This article is intended for our colleagues who install whole-home generators in Southwest Florida. Licensed electricians often don’t realize they are creating a hazardous situation that could severely damage equipment. Warranties for generators or solar inverters can be voided by failing to take into account important factors. We have seen this time and time again, so we wanted to point out a correct way to integrate a grid-interactive solar energy system with a new whole-home generator.
Whether you are installing a new whole-home backup generator with solar panels, or whether you are adding solar panels to a home with a backup generator, this article may apply. There are too many scenarios to explain every one, but this is the most common question we come across.
The bottom line is you cannot allow solar panels and a generator to work in parallel. They must be electrically isolated at all times. If solar inverters “see” voltage from a generator, they will attempt to sync with the generator and backfeed power to it. Any time solar production exceeds loads in the building, solar inverters attempt to send power to the utility grid. As a huge “battery” of sorts, the grid can handle this small amount of backfeed. Typical residential standby generators cannot.
Please watch this video for more information:
It is important to note that solar panels cannot be used in a utility outage without batteries. Solar panels also can’t work in parallel with a generator to reduce fuel usage. These are two common consumer misconceptions that you need to be aware of when you encounter a client that has existing solar panels. There is a lot of misinformation floating around, shady solar contractors, and lack of consumer education on solar energy.
If you are in our service area, we would be happy to assist you with re-integrating a solar panel interconnection with your generator installation. If you need advice prior to bidding or installing a standby generator or a critical load generator, do not hesitate to contact us. We partner with electricians to ensure a smooth customer experience. Ultimately, it’s in our best interest to work together to keep homeowners safe and equipment operating properly.
Contact us today to discuss how we can help.
I don’t have a comment, but a question about whole house generator and solar. In reading your comments on where to connect the solar input to the home you say connect the solar input on the line side of the Transfer Switch. Without a generator the solar breaker is in the main electrical panel and as far from the main breaker as possible. If it tied into the Transfer Switch Line side, it will be on the input side of the main breaker panel. Will this overload the transfer switch’s main breaker? Would you mind explaining this in more detail?
An overload on the transfer switch’s main breaker can only come from the load side (an overLOAD). You could technically have a 1,000A utility service to your generator transfer switch and a 200A main breaker inside of it. That never happens from a practical standpoint, but it is technically possible.
The supply to the main breaker has nothing to do with it. If you connect your solar inverter output to the line side of the transfer switch, by code it cannot exceed the ampacity rating of the utility supply. You would not want to feed more solar amperes onto the utility conductors than they are designed to carry. For example, if you have a 200A utility service and there is no load in the house and you connect 200A of solar inverter output, the entire solar supply will be fed back to the grid. Now in reality, many utilities have lower limits. In Florida, it is 90% of the service rating typically. Regardless, that is still a LOT of solar capacity. a 180A solar energy system could be 60kW of solar capacity – enough to offset a $900 electric bill. If you have a $900 electric bill on a 200A service, please call me. Something is definitely wrong!
You say that without a generator the solar is connected to a breaker on in the main panel at the opposite end of the bus bar. This is not necessarily the case. Current electric codes allow the breaker to be in other positions on the bus bar in some cases, or on feeder conductors if certain requirements are met. But in many cases, even without generators, the solar energy system is connected on the line side of the main disconnect. Many times the rating of the solar energy system exceeds the code-limited capacity (ampacity) of the bus bar. Around here, most main panels are 200A and the main breaker is 200A. In this case you can only have 32A of rated inverter output connected (around 7.68kW). Many of the systems we install these days exceed that rating and must be connected to the line side of the main disconnect.
I would estimate that 70% of the residential systems we install are connected on the supply side of the premises main disconnect now. That is likely to increase in the future.
P.S. This gets complicated if you have a meter/main combination device. We are seeing a lot of contractors reduce main breaker ratings from 200A to 150A to allow a larger load side connection. While it may comply with code, it may not be in your best interest in the long term. We can help you with solutions that don’t take this shortcut and that consider your best interest. The short answer is – hire a real pro that doesn’t take shortcuts and knows what they are doing.
Great discussion on the topic. Solar company down here in Mexico made the mistake of hooking up the panels as described as the “don”t” install with a whole house back up generator. No damage so far. My solar saving are not what they are supposed to be and I suspect that would be part of the issue. We have had the generator in the off position since I saw your video. Thank you for the information. As you know, the generator company starting with “G”, allows very little information online regarding their systems and installation advice. My question, we have installed a disconnect with “breakers” as you show in your drawings. Should we be installing “fuses” instead? BTW, dealing with 3 phase power down here in Cabo. Is it important to tie all three 120 volt leads with the solar outputs wires? in the breaker box they now have solar going to all 3 phases with two breakers double lugged. It is Mexico, that is allowed, but will be corrected when we get it on the other side of the transfer switch. Thanks for the help……
First, circuit breakers may be acceptable as long as they meet the current interrupt rating required for your installation and as long as they meet whatever building code you are trying to meet.
I’m not quite following how your system is connected by “double lugging” breakers, which may or may not be allowed, but is definitely a bad practice as inverter output should land on dedicated overcurrent devices. And you should balance your inverter output across phases (or better yet, use 3-phase inverters). If you do not balance the PV output, you may not get the billing benefits of reduced demand charge, but I have no idea how electricity is billed in Mexico. You should really seek the assistance of a licensed local electrical contractor that is experienced with solar.
Thanks for the helpful information!
Can you update your natural gas generator to solar or do you need to get a whole new system
A gas generator is a completely different fuel source and cannot be converted to solar. Are you possibly talking about adding solar pool heating panels to a pool with a natural gas pool heater? If so, you definitely can have both.
During power outage if I turn off the solaredge inverter and also Utility disconnect, before i start Generator, will I still run into feedback issue.
Preet: No, you will not have a feedback issue, but it is illegal. You should install a proper transfer switch or interlock to prevent accidental problems.
So, I have a 2ndery home up north with no utility power, but I do currently have an Onan 9500dpf generator running the power in the house. I am currently installing a solar system to be the main source of power to the house, but still want the generator to turn on as a backup to when the batteries get to low. How and what would I use to achieve this? Thanks
You can install any number of inverter/charger hybrids that support automatic generator start. If you are unfamiliar with this, I suggest you seek out a qualified solar professional that specializes in off-grid systems. Trust me – I wasted huge sums of money doing things wrong when I was a DIY off-grid solar owner. It’s complicated and there is huge value in hiring someone with deep experience. But there are too many variables to provide you with a specific brand/size/kit that will work for you. And the off-grid kits you will find online may not be suitable for you, either. That’s where a professional will provide value for your project.
How can I use solar panel to generate with out fuel.
You would need batteries and an inverter.
When I lose power my solar does not generate any power as I do not have a battery back up. I was hoping a generator would be able to act as incoming power and allow my solar to function. Is this not doable?
You cannot do that. You need batteries as a buffer (and a hybrid inverter). Think about it this way – what would happen if your solar panels were producing lots of power and you had little to no load in your home. Where would the power go? It would try to backfeed the generator, thinking it is the grid. Bad things would happen. In theory, someone could design a system of relays to sense power needs and make this work, but because loads are intermittent and could be shut off abruptly, it’s not really practical. There needs to be some kind of buffer. At this time, there is nothing on the horizon that can do this in a safe manner.
Thanks for all your useful comments. I have a grid tied system for solar saving and a generator for solar backups.
I noticed my solar grid tied system continues to run while on my generator. They are meant to shut down when running on generator. What damage could have been caused as a result of such. I have started to notice strange sounds coming from the generator and lights flickering.
That’s exactly the point. It is unknown and unclear what damage might occur. Because generators and inverters are not designed to work together, they should not be coupled unless it is done with a purpose built, UL listed and tested integration system of some kind.
I would get that installation error fixed as soon as possible. In the interim, turn your generator off manually and do not allow it to run when your solar panels are working.
How do you isolate utilities from solar power?
By code you have to disconnect it during outage otherwise you will electrocute workers working on the grid. Manual fuse won’t do it, need to have some auto switch solution.
There are two basic kinds of solar energy systems. One is strictly grid-interactive and does not provide any backup function when the grid goes down. In this type of system the inverters shut down automatically and no additional switching is necessary. In a battery backup system, you do not necessarily need an automatic switch. There are various types of hybrid inverters that provide internal automatic transfer switches, but it is perfectly acceptable to have a manual switch that allows you to power your home with solar (and batteries) as long as it does not backfeed the utility grid during an outage. There are numerous ways to accomplish this depending on the type(s) of inverters in the system.
Hi, Thank you for the useful information. I have a question.
I read one of the reasons why we need a interlock kit to install the generator, is it eliminates unsafe generator backfeeding situations that could potentially electrocute power linemen.
But when we have a solar system, when there is an outage, the power from solar system will go to the grid, is that some kind of safety concern for the linemen?
See my response to Andrew. This function is handled automatically in a grid-interactive system. The inverters are listed to UL1741 which specifies an anti-islanding function that prevents solar panels from backfeeding the grid when there is no grid voltage present.
Am I correct in believing that no power would be produced from the solar array if the grid is down because it needs to sense a voltage when connected to the mains side of the isolating contactors – something to do with islanding in the invertor.
That is correct. A grid-interactive inverter that is UL1741 listed will cease to operate when it does not sense voltage on its AC output.
Jason you are a saint to answer some of these questions without telling some of these guys to stop where you are, put your tools on eBay, and never never come within 6 feet of any panel ever again.
Yeah, that’s why I have the strong disclaimer at the beginning. You would not believe how many calls I get on this.
talk about bad solar installers 2 neighbors 2 blocks up got panels. one got all panels pointed south. the second 80% of the panels face east flat on roof. and a few facing due south and 0 on the west roof. cannot imagine that one getting any ware near what the full capacity.
You might be surprised, but in Southwest Florida, panels flush-mounted on an east roof are only about 8% less productive than an ideal south roof. And west is slightly less productive than east (due to heat and afternoon clouds). It’s a misconception that solar panels must be oriented or tilted south. More information here: https://floridasolardesigngroup.com/solar-electricity-output-based-on-tilt-and-orientation/
I’m a solar owner with a SolarEdge inverter (HD Wave). The original inverter failed after 9 months (the replacement has been OK for 10 months). I’m now thinking about getting a Generac gas powered generator. I’m worried about the generator ‘frying’ the inverter either during the weekly self-test or in an actual power outage. If the generator is properly installed by a reputable certified installer, should I be worried? Since one inverter has already failed, if it happens again after installing the generator, I worry that SolarEdge will void the warranty even if the generator was properly installed.
A “properly installed” generator when you have solar panels is completely electrically isolated from the inverter. There is no chance of damage. But to do this properly you need a transfer switch, manual or automatic, that completely disconnects the solar from the generator during operation. The solar panels should be installed on the utility (line) side of the generator transfer switch or manual interlock system.
Don’t take for granted that your generator company knows what they are doing. And don’t trust your building department to be a last line of defense. Direct your generator company to this article and/or get your solar contractor involved to ensure it’s done correctly.
I haven’t read through all of these comments, but this one seemed to align closely with the question I had after watching the video. If the utility company has an outage and fails to provide electrical service to your home, isn’t this part of the reason for having a solar solution? I would think any generator would be purchased with the intention of it being a backup to the solar power system. Why would you want to disable the solar in the event of an outage from the utility company?! I feel like I’m missing something.
Grid-interactive solar energy systems are intended to offset utility electricity usage and have no backup component. They can be paired with a battery system and multi-mode/hybrid inverter to provide a backup element and integrate a generator for auxiliary backup power. In some instances, these multi-mode inverter systems can use a system controller to parallel generator output with solar production (like the Enphase Ensemble backup system). Generally speaking, there is little reason to operate solar panels in parallel with a generator. You will not save much fuel (if that is the thought you had) and the gear required to parallel systems is very expensive. Using a generator as a redundant backup system or auxiliary to a battery backup system is often the preferred route.
Nonetheless, this article focuses on straight grid-interactive solar energy systems with no backup component where a generator is the only source of power for the home in a grid-out scenario. This is by far the most common and cost effective strategy in most cases.
Hello, thanks for the usefull information.
I have a couple of questions.
-Some inverters are equiped with a commutation contact. ¿Could I use this to send a signal to the inverter from the diesel generator to disconnect solar system when generator is on?.
-If solar system’s power is low compared to diesel generator (solar system= 20kW; diesel generator= 630 kW) could I have the same issue of compromising the generator if solar wont’t disconnect?
I have a couple of questions:
1 – Some inverters are equiped with a commutation contact. Is it possible to send a signal from the generator to the inverter’s contact so the inverter disconnects when the generator is on?
2- In a system where solar power is low in comparisson to diesel generator (solar system= 20kW, diesel generator= 600kW) the risk of backcurrents to diesel generator can be neglible?
We do not recommend using relays/contactors to disable the solar output because it is not a failsafe solution. In theory, it should work, but I don’t believe it is good design practice when there are other alternatives using a double pole switch (automatic or manual) or a bypass assembly that physically disconnects conductors.
The relative size of the solar and generator are not the primary factors. The relative size of the load to the solar production would be more important. Since you can’t guarantee a load, you should not expect the load to consume all of the solar production. That would be poor design practice.
Great info. I have a gas powered generator 5000w/6500w that I have can plug into a transfer switch. When the grid goes down during a hurricane, I turn off power to the grid and plug the generator into the transfer switch. It generally is good enough to fire the LRA on the smaller of my two A/C units (4 ton), as well as run the fridge. My stove is gas, and I don’t do laundry. I’m looking at a 10.5 kW solar array with battery backup. The LG RESU10H. I would like to power the a/c at night with my generator while running the rest of my home with the battery backup until dawn (or whatever) while (hopefully) simultaneously charging my battery. Is this possible?
Possible? Yes. Advisable, most likely not. There would be substantial costs associated with separating loads and then you are very limited on what you are backing up and charging. Why not use your solar for everything you want to backup and just use the generator as an auxiliary energy source to charge the batteries when solar and batteries are insufficient?
Running a generator to power an inherently intermittent load is incredibly inefficient. The generator will run all night and the air conditioner will turn off and on.
What you should really be considering is a traditional hybrid inverter that has dual AC inputs (grid and generator) with an integrated transfer switch and integrated battery charger. Something like a Sol-Ark 12k would be a great option. Then get a 48V LiPO battery bank. You probably need more battery power than you are estimating (everyone underestimates this), but you could install something as small as 10kWh. You would have less generator run time as your battery capacity increases. Two Fortress eFlex 5.4kWh batteries would work. More would be better. The Sol-Ark would manage everything, turning the generator off and on as necessary (provided that it has remote start capability). If your generator does not have remote start, you could still manage the generator runs manually.
The LGRESU10H (and a Tesla Powerwall 2) only has a 5kW AC output rating. That probably will not start your air conditioner. But a Sol-Ark 12k most likely will, even with the same amount of battery storage capacity. Just use the generator to recharge the same battery the Sol-Ark is using. It’s way more efficient to do it that way. Don’t try to separate systems.
The solution is right in front of you. We have had commercially available grid-tied solar with battery backup and generator support for over two decades. There is no need to reinvent the wheel (yet many are trying). The Sol-Ark inverter even does both AC coupling and DC coupling for solar. It’s a very flexible solution. It would do everything you want, maximize flexibility, and improve efficiency.
The big “AC battery” companies (Tesla, Enphase, Generac, LG, etc.) are all trying to introduce generator support to their battery backup solutions. While these systems seem to make sense for their simplicity in AC coupling the battery output, you also have to ask yourself what happens if one of the components fails. What happens when the batteries reach the end of their useful life? A traditional hybrid inverter system allows you to change batteries because the inverter is battery agnostic (chemistry, size, type, etc.)
We are already seeing Tesla Powerwall owners who want to increase their capacity and have no options, either because Tesla no longer produces the original Powerwall or they won’t sell the Powerwall 2 anymore unless you are buying a new PV system (reportedly), even if you already have a Powerwall 2 system.
There are plenty of ways to make this all work with various brands. Outback and Schneider both make well-established hybrid inverter solutions that can use AC coupled or DC coupled solar. SMA can accomplish this, most easily with an AC coupled Sunny Boy and Sunny Island solution.
At this time I would stick with a traditional hybrid battery backup inverter with dual AC inputs. Whether you do AC or DC coupling boils down to cost and other factors like shading, warranties, and personal preference.
Just an FYI,
Schneider is not the only company that has an issue with getting generators to play nice with AC coupled PV. See sol arks wiring diagrams. [link deleted] Notice that they all list the generator input as “Gen or A/C coupled PV” meaning you can’t have both at the same time. If you A/C couple on the load panel then they say “DO NOT USE” for the Gen input. See page 10 diagram 4.
If someone is doing a clean install and plans to have a battery backed up system with the ability to use their PV and Generator when off grid, DC coupling is really the way to go. The only AC coupled system I am aware of that can do this is a complete Enphase solution. I don’t know about all Hybrid inverter companies, but if neither Sol Ark nor Schneider handle this, I would assume that most other vendors don’t either.
It’s important to note that you cannot use your generator and PV “together” in a DC coupled system the way that many think you can. In this configuration, people think that the generator is going to charge the batteries, and the inverter and PV will continue to power the house loads. That is not the case. When the generator runs, power passes through to power loads directly. The remaining generator power is used to charge batteries up to the limit of the inverter’s integral charger. However, this can be inefficient when the generator runs at the same time as PV is available because the system will have a maximum charge rate, wasting the PV power if the generator is charging the batteries at their maximum rate. This isn’t a big deal for system that are rarely off-grid, but just a small point.
There is also the question about whether you are eligible for a tax credit on the batteries if the system can be charged with the generator. The tax credit is intended for batteries that are recharged solely by solar. In practice will the IRS come knocking to check this out? No. But it is technically an issue to consider.
There are no perfect answers right now. If you want solar with battery backup and auxiliary generator support, there are AC and DC coupled options out there. Most people should opt for just batteries or just a generator for backup purposes, even if they have solar panels. The redundancy is expensive and complicated. But if you want it all and can afford it, we help people find that ultimate peace of mind all the time. For people on a more modest budget, consider how badly and how often you really need backup, and invest in a solution that just meets those basic needs. Or, as Michael suggests, just bug out to the Ritz when the next storm approaches. It’s probably much cheaper!
Timing on the topic is amazing. We are in the planning phase of construction. An existing remote pole barn is where we want mount the panels with Enphase 7 inverters for a 5 kw to 6 kw system and a Generac 14kw generator for emergency power on a grid tie system . Neither Enphase nor Generac would assist with technical support on the proper way to interlock the systems. You seem to have answered this for me, I think.
If we have the utility set the meter at the pole barn the line voltage would feed the Generac transfer switch and that is where the Enphase parallel to the meter connection of the transfer switch? The Gererac connection is on the other contactor of the transfer switch.
From the transfer switch we will run line service to the main service panel at the house.
I’m in Michigan and how do I find a company to engineer a system for a DIY install?
You have the right idea. Get an appropriately rated service disconnect installed on the line side of the transfer switch for the solar when you install it. You can do this in a variety of ways. If the transfer switch has double lugs, that’s the easiest way. Polaris taps or insulation piercing taps also work.
Most sellers of solar equipment online will be able to provide you with engineering drawings for a fee. You may need a signed and sealed set of drawings. Often the harder part is getting structural drawings for a DIY installation. You can reach out to local solar dealers, but most won’t help DIY installer for obvious reasons – you are competition. DIY installers also tend to require a lot of hand-holding. Ultimately, you just need an engineering firm that specializes in solar and is licensed to work in your state.
Also, note that some states and jurisdictions prohibit DIY electrical work. You may need a licensed electrician. I would highly recommend that you let someone qualified look over the work at a bare minimum, even if your jurisdiction allows owner-builder permits for solar energy installations.
Thank you Jason! Today I took, as you wisely suggested, the step to contact a residential structural engineer (for a fee) to make sure the array will meet loading requirements with the roof trusses that are existing. In the midwest we do get heavy snow and the panels don’t add much weight, but pole barn trusses might not be adequate without modification. Good call and thank you very much!
Jason – I apologize for asking what amounts to same question yet again in a slightly different way, but I just can’t find the answer anywhere else.
Prior to adding a solar/battery system, I had a 6 circuit manual transfer switch (3 position switches with line/off/gen) wired into my panel and connected, when needed, to a 5500 watt portable generator for times when the grid dropped out. In January I added solar panels and 3 Tesla Powerwall2 batteries. The incoming power grid line, battery, and solar/inverter all go to a Tesla Gateway which then sends the power to my panel. They didn’t touch the MTS at all during the installation. Can I assume that the MTS still does what it did before, i.e. when I move any of the transfer switches from line to gen it isolates that circuit so that there is no possibility for me to feed back from the generator to the invertor or battery and there is no possibility that the invertor and battery can ever feed back into the generator? I’m hoping I never need the generator, but with enough overcast days in a row, I could empty the battery. I read your previous posts and being able to recharge the battery from a generator would be great and I wish the installer had offered it as an option, but I’m happy if I don’t need to spend any more money and am no worse off during a streak of bad weather than I was before. Thanks for your comments.
First, don’t assume anything. You should have a licensed electrician look at the wiring and ensure it is done that way. If it’s not, it’s not done correctly.
But I would be willing to bet that what you are saying is correct. You should still be able to run your generator for those protected/critical/backup loads without backfeeding the solar inverters and batteries. If they didn’t change anything with the manual transfer switch, it is highly likely that it will operate as it did prior.
Excellent information and discussion adding further clarity.
Curious, if one did not have a whole house generator but rather a generator for a protected loads panel, could you still interconnect the pv on the load side in the main panel or sub panel down stream and keep the pv off and isolated during a grid outage while the generator feeds just the protected loads panel?
Yes, you can do this if you have a properly installed transfer switch for your protected loads panel. See my prior comment to Steven Feldman. I think that’s exactly what he has.
Have you heard of Chilicon micro inverters? They claim you can use them with a generator without batteries. They are smart enough to lower production as needed. [link removed]
Under application white papers, see AC coupling with chilicon micro invertors.
What do you think? Should I try it?
I am aware of the claim, but the documentation is pretty thin on details. Having discussed this with a trusted group of colleagues, we are not aware of anyone who has actually tried this and we don’t advise it. Most generator manufacturers we are aware of state that their warranty would be void if inverter output were to be paralleled with generator output. I’m not sure how much fuel you would save, anyway.
The problem with paralleling inverter output with a generator is that you can’t predict when a heavy load will go away. If you switch off or disconnect a large appliance, the inverter may not shut off quickly enough, and then what happens? It’s unpredictable and therefore unsafe. You need a buffer of some kind (a battery, for example).
We could be convinced and proven wrong, but I think we need a lot more data and experience in the industry to support this kind of claim. It’s definitely not being used widely.
I live in California (Bay Area) and is served by PG&E. Last couple years, the wild fire seasons prompted the utility to shut off the grid for multiple days. Hence, we added a whole house generator (Generac). Now we are toying the idea of adding solar to power our house and cars. Our generator was installed by licensed contractor. Are there any good solar companies that can put all theses together?
We can’t recommend any contractors in your area, but this should be easy for a qualified solar contractor. Integrating solar, batteries, and generators has been happening for decades and there are lots of new options to make it even easier than in the past.
Change your generator frequency to above 62 Hz and your PV inverter will see the grid is out of spec and will not connect to the grid while your generator is on. Once the utility power comes back on, generator shuts off and PV inverters will reconnect. Easy solution without expensive generator controls.
We do not advise this strategy. It is not a failsafe method and I’m not aware of any generator manufacturer that instructs this installation method. It would likely void the warranty. While it may work, it’s not a tested and documented way to interconnect a generator and inverter output.
I don’t know how failsafe you want it to be. Enpower says that frequency trip is /required/ by IEEE 1547 and indeed that is how batteries are designed to work. per Enpower:
“Many grid-forming, battery-based inverters have the ability to shift the operating frequency in off-grid mode when their batteries are fully charged. Setting the battery-based inverter
over-frequency shift setting to a value above the microinverter system local interconnection frequency trip point, will cause the microinverters to completely stop producing power”.
They go on to describe that only 60.5Hz is enough (the IEEE requirement) to trip the PV inverters.
We went solar in January 2021 and I would like to have a 10 circuit transfer switch installed by a licensed professional. My fear is that most electricians do not have the solar knowledge as you describe and would potentially incorrectly install the transfer switch where it would destroy my inverters. Do you have recommendations on whether to have a general electrician or would my solar company be able to install the transfers switch correctly. Thanks in advance
A licensed electrician should be able to figure this out quite easily. This article is designed to help qualified professionals who might have some questions about how to do it correctly and safely. If you show them this article I imagine they should be fine.
I am building a new house with a whole house generator with a transfer switch. After I move in I would like to install solar – not to provide power to the house, but only to sell back to the power company to offset my electric bill. Could I easily isolate my generator from the solar since I dont plan on using any of the electricity coming from the solar cells?
Yes, it’s relatively easy to integrate solar panels to do this. However, that’s not exactly how it works. Solar energy will be used by your home first to the extent there is a demand. Only excess energy produced will be sent back to the grid.
How this is accomplished depends on what your utility requirements are. Some places use two meters to handle netmetering. Some (most) use a single meter. Others may not allow netmetering at all. Each state and utility has their own rules.
Regardless, connecting solar output between the transfer switch and the utility is what’s required.
Thank you for all of the information. The article was very helpful.
My solar system has 2 switches and a breaker between itself and the panel. There is a large switch on the inverter box itself, a second fuse style lever switch between the inverter and the panel, and of course the 60A breaker in the panel. If all of these are set in the off position, can I assume that a generator will be able to power the essential circuits of the home without damaging anything?
Jason, many thanks for the clear and crisp advice above – other advice for integrating home standby systems with solar systems gets very confusing.
Reading your responses, if I want to avoid the startup and ongoing costs of battery storage, and want to add solar to an existing (Generac) home standby/automatic transfer switch system, I think I will end up with two operational modes:
1/Grid up with solar feed
2/Grid down with generator feed
Is this a fairly simple job for a qualified solar technician to install?
From my experience when solar is on the dedicated loads panel of a backup generator and the grid is down with the generator going, the solar stays in a search mode looking for the grid and is not back feeding. Doesn’t the generator’s voltage rise when the solar tries to send power and that is what keeps it in the islanding mode. I do not believe this event will hurt the generator or the solar since they are doing what they are designed to do.
I have only seen this on small gas generators but is there a reason it would not work with the larger whole home back up generators or is there something that I missed. Because I can not find a manufacture of either inverters or generators that really have any good documentation on any of this other than saying they do not support it and it could void the warranty.
Please pass on any documentation you may have because we are in the fire areas of California where we are wanting to add a generators to our home and the solar is on a downstream subpanel that can not be left off of the dedicated sub since it has a well on it.
If the solar panels and inverter are connected to the utility side of the automatic transfer switch and there is a utility outage, the ATS will disconnect the utility side and the house will run 100% off of the generator. I get that completely and it makes absolute sense.
My question is whether the solar system will be backfeeding power up the utility lines. We live in a hurricane prone area and don’t want to electrocute a lineman working to establish service. Is there a way to prevent that as well? Where does the engery created by the PV cells go if they are isolated completely?
Phenomenal information here by the way.
Does the Sol-Ark 12KW require a separate
PV inverter for an AC-Coupled / Off Grid type of system with battery back up.
I am looking to produce AC power from PV panels without the grid and would like to operate PV inverter as the primary function and battery inverter as the secondary.
I hope that makes sense.
I am Off grid completely and have
a 36KW Multiquip Diesel generator that
I am looking to replace with off grid solar.
To be used in the new system I have
(2) Honda Eu7000is inverter generators paralleled and 48V / 240ah LiFePO4 battery bank (x2)
I’m trying to decide between SMA Sunny Boy/Island combination and Sol ark 12Kw. In your opinion which offers the best system options for being completely off grid? I want to verify whether the the
Sol ark requires an additional PV inverter similar to the SMA sunny boy in order to produce AC directly from panels or that it does it all?
The Sol-Ark does it all. While you can AC couple with it, you don’t have to. It includes DC inputs for up to 13kW of PV. You don’t need charge controllers or any other gear (other than disconnects and overcurrent protection).
The Sol-Ark will produce AC power from the PV directly and excess PV power will charge the battery. It’s the nucleus of the system. For a super redundant system, you could consider dual paralleled Sol-Ark 12k, or if your needs are modest, dual 5k units.
If your Hondas can auto start, the Sol-Ark can manage that as well!
Thanks for the clear information, and excellent video. I have a small (5KW) system that is back-fed through a circuit breaker on the load side of my main box, like in your video. I would like to get a 5.7KW Generac natural gas generator installed. It comes with a 50-Amp automatic transfer switch, which has 8 circuits on its panel. It looks like that would allow me to keep this arrangement for my solar energy since only those 8 circuits would receive power from the generator, and all the other circuits (along with the back-fed solar breaker) in my main panel would be completely cut off from the generator’s power when it switches on. Am I thinking clearly on this?
> The bottom line is you cannot allow solar panels and a generator to work in parallel.
Enpower says you can, using their smart switch.
I have a 5Kw chilicon based solar system. They advertise that you can hook up a generator and save fuel. It does have no export settings for places that don’t allow exports. Do you know if this system will work with a genset at the same time.
I have seen that advertised, but the details are minimal and I have never personally heard of someone successfully implementing this. First and foremost, generator manufacturers don’t support it as far as I know, and most specifically say it will void their warranty. If you have a way to keep PV output less than the connected load at all times, I can see how this can work. Enphase’s Enpower Smart Switch allows generator use with PV, but that system also has a battery to buffer the system.
But more importantly, how much fuel do you really expect to save with a residential generator. Usually, a generator in a residential setting works at a fraction of its maximum output and the load is very variable. The difference in fuel burn would be minimal or even negligible by having a PV system offsetting whatever it could, but the PV inverter would need to shut down when loads disappear and would not reconnect for about 5 minutes once loads reconnect. This would be very ineffective. The only scenario I see where fuel burn could be reduced is where the load is consistently, and perhaps significantly, above the amount of PV available.
For practical purposes, it doesn’t sound like a sound strategy. Using a battery does, albeit expensive. I appreciate people pushing the boundaries and trying to reinvent the wheel, but a battery-based system that buffers energy use and integrates PV and generators is still the best way to go, the safest, and the most practical in my opinion. Maybe the Chilicon people can convince me otherwise with more information and some real-world applications where this is working safely and effectively.
I am very impressed with your knowledge and practical advice!
I have an off-grid whole house power system in the USVI. The solar array has thirty panels each with its own microinverter and four Powerwall2 batteries; array and batteries are controlled by a Tesla Gateway controller. The Gateway has inputs for grid power and generator, but neither is being used. Tesla says that my generator – a Kubota PDF20 manufactured by RK Power – is not approved for use with the Gateway but does not give me a reason. (“It is not on the approved list.”) House load is connected to an automatic transfer switch (ATS) that selects power from either the Gateway or from the generator. The house is normally powered by the array/battery/Gateway system, but when the battery charge is depleted to 5% the generator turns on and the ATS selects generator power. All of this works fine.
The problem is that in order to return to solar power I have to manually restart the system. It works like this: Typically the battery depletion occurs at night; typical load from house is 1 kW. When the sun rises the array begins to produce a few watts of power and the Gateway attempts to switch back to solar power. After “multiple tries” the Gateway shuts down the batteries. House load continues to be supplied by the generator.
The manual restart procedure is first to turn off the ATS so that the Gateway sees zero load from the house. Next the four powerwalls are restarted (by turning off and on the toggle switches). The intent is that with zero load from the house, power from the array will charge the batteries. In practice the gateway itself consumes power – typically 100 – 400 watts, and as the batteries are already depleted, the array must produce at least this much power in order to prevent the gateway from shutting down the batteries again. In the early morning or cloudy days the restart procedure may need to be repeated several times before the batteries have sufficient charge to power the gateway without solar power. Once the batteries have sufficient charge to avoid being shut down by the gateway I continue to recharge the batteries – with zero load from the house – for several hours before turning on the ATS at which point the house is again powered by the array/battery/gateway system and the generator turns off.
I am looking for a way to restart the system automatically. This may seem like a trivial problem, but requiring a manual restart means that the house can not be left unattended. (The generator will run out of diesel eventually.)
In my view an ideal solution would be that when the batteries are depleted that for some period of time – say two hours – that the generator would not only supply power to the house but also charge the powerwalls. After that period of time the generator would shut itself off and the house load would be satisfied by the array/battery/gateway. I am told by Tesla that this is not possible.
I am open to any solution.
Thank you in advance for your help.
Unfortunately, I don’t believe there is a way to restart the system automatically under this scenario. It is one of the deficiencies of the Tesla Powerwall system in an off-grid scenario.
Thank you for all of this good info. I want to integrate a portable generator and was planning on an interlock approach, avoiding a separate (and pricey) xfer switch by using a sliding mechanical device that prohibits the main and generator breakers being “on” simultaneously. This means I would not have a separate transfer switch. Looking at your drawing, is my planned approach even possible (advisable). I’d planned to juggle the circuit breakers myself during the outage (poor man’s transfer switch). Thanks
Sorry to have bothered you with my previous question. After more reading I see that I can interconnect to the supply side without a transfer switch, so the left box in the drawing would disappear, the combined inverter and utility power would come into my panel via the main switch, and I would connect the generator to the panel via circuit breaker. A mechanical interlock would separate the utility/solar power from the generator power. Thanks again for this article – it helped me get over my confusion about how to manage things with inverter AND the generator coming in via circuit breaker. You needn’t answer the previous question unless you think it would be helpful to other readers.
No problem. A mechanical interlock is technically the same thing as a transfer switch. As long as your solar interconnection is on the line side of the mechanical interlock, you don’t have to worry about your generator. However, there may still be code issues to consider depending on whether you are doing a load-side feeder tap or a supply-side interconnection. That is beyond the scope of this article, however.
There do seem to be breaker panel interlocks designed for solar. They prevent both the main and solar breakers from being on if the generator breaker is on.
A generator interlock prevents the main breaker and generator breaker from being on simultaneously. It does not prevent solar from backfeeding a generator unless the solar is interconnected on the line (utility) side of the main breaker, which is exactly the point of this article/video.
In fact, we see people installing generators with interlocks frequently, thinking they are protecting their solar energy system and generator properly, when they are not. That’s why you have to follow the suggestions here.
We have installed our solar system a few weeks ago, while keeping the generator on for night use and batteries for when we sleep. Although we have given priority to solar energy before the generator, the screen of my inverter indicates that the solar system is feeding our house through the batteries during the day and that the generator is in bypass mode at the same time.
Could it be that both solar panels and generator are giving us electricity at the same time?
An engineer installed our solar system but I don’t quite understand the inverter’s screen.
I would need more details and specifics, but it is unlikely that the type of system you are describing would parallel the inverter and generator output to your home.
excellent informative discussions – If I am following your notes – I built my new house with the utility meter serving a panel, a breaker from that panel feeds a transfer switch and a whole house generator. I left extra breaker slots on the panel under the meter upstream of the transfer switch. My understanding, is If I put a set of solar panels (no battery, just net meter to off set demand) feeding into one breaker – if I have a power outage, the transfer switch cycles separating the generator from the utility source panel, at which point the solar panels sees a loss of grid and shuts down due to a loss of frequency/voltage from the grid. When the grid power returns, the transfer switch cycles and disconnects the generator and the solar inverter restarts. So the generator and solar should never be in parallel. Is my logic correct? thank you
It sounds like you have that right. Depending one the size of your PV system, you may or may not use that breaker space to backfeed inverter output to the main distribution panel. A supply side interconnection could also work.
I’m an electrician and Generac dealer. We have a customer with an off grid cabin that we have wired and are almost to the point of finishing. He has bought solar kit with batteries and wants me to install it. I haven’t seen the kit so I’m scarce on details at the moment. I told him I knew very little about solar but he is insisting that I look at it. He has sent me a detailed schematic and I can probably but reluctantly get it installed. I know from past experience that you need a backup generator to keep the batteries charged on cloudy days etc. I also know that the generator has to be compatible with the inverter in order for it to start and stop the generator. I told him he needed a Generac EcoGen 15 kw unit for backup. Because of covid and everything else Generac is months out on all units. He didn’t want to wait so he bought a propane portable generator. I told him I don’t think it will work because it doesn’t have the technology to work with the inverter as far as starting and stopping. He said he would start it manually when needed. My question is what will happen when the batteries reach full charge from the generator and he doesn’t shut it off in time? Will there be a back feed and damage the system? It sounds like a recipe for disaster.
“Scarce on details” makes this hard to answer of course, but it might not be as scary as it sounds. Generally speaking, what you are describing is probably a solar with battery backup system that uses an inverter/charger combination. There would be an integral transfer switch in the inverter/charger. The generator would connect to the generator AC inputs and the charger would draw generator power as required to charge the batteries while simultaneously powering the loads. The transfer switch passes through generator power to the loads when generator AC input voltage is present. If the batteries are full, the charger simply would not draw anything from the generator. Most inverter/chargers of this type will back off the charger if the loads are too high. You specify the max generator amps and if the combination of loads plus charging exceeds this, the charger will draw less. If the generator remains running and the batteries are full, the charger shuts off and loads continue to be directly powered by the generator. As soon as the generator is shut off, the transfer switch reverts to inverter power to carry the loads.
Good day Jason. Thank you for your kind expert advice. Did I miss out the reply from …
AUGUST 27, 2021REPLY Jonathan Godfrey
Thank you for all of the information. The article was very helpful.
My solar system has 2 switches and a breaker between itself and the panel. There is a large switch on the inverter box itself, a second fuse style lever switch between the inverter and the panel, and of course the 60A breaker in the panel. If all of these are set in the off position, can I assume that a generator will be able to power the essential circuits of the home without damaging anything?
I have a transfer switch installed to plug the 7k watt portable genset to feed my home its basic needs. My solar feeds excess back to the grid. Do I need to switch off the mail breaker for the solar inorder to run the generator safely? Thank you, Stay safe and happy holidays.
What you can do and what you should do are two different things. It does not sound like you have a safe and compliant installation as-is. You need to make it failsafe using the information presented in this article/video. I would not attempt to do what you are suggesting, just by virtue of the fact that you have to ask. If the average (or below average knowledge) homeowner is able to cause severe damage doing something that is seemingly safe, it is not installed right. Hire someone that knows how to properly isolate grid-interactive solar from your generator.
I have a 16 panel 540W installed along with 12 batteries 12V 220Ah each. Both connected to a solar hybrid inverter. I am trying to use the generator of the house which is of 13KVA as an AC input (utility) for the inverter to charge the batteries and supply the house. But when the generator is running through the inverter to the batteries and the house I am having high voltage fluctuations in the house. Why is that and how can I solve it?
Thanks in advance
With the limited information I have, I would guess that you need a better and/or bigger generator. In this scenario, usually the inverter charger is just passing through generator power to the house. The inverter is not providing the power anymore. Simultaneously, the integral charger is drawing power from the generator to charge the batteries. This creates a heavy load on the generator, which could explain the voltage fluctuations. You could try reducing the charging current that your charger draws and see if that improves the situation.
Numerous times you comment, suggest or state that solar panel power is connected to the LINE side of a home transfer switch such as in your comments to Jerry Liddle.
In my case my utility power comes from my electric meter to a transfer switch (Line Terminal) that is also fed from a whole house backup generator connected to (Aux Terminal). This transfer switch is mounted between the electric meter and the house breaker panel. House breaker panel is (Load Terminal).
The “utility power” is monitored by the generator and when it senses the utility power is off it starts and automatically transfers the house load to the generator. You cannot connect any source of power, backup generator or solar to the Line side of any transfer switch that is mounted between the meter and house breaker panel. This would create a backfeed situation to the utility and create a deadly situation for utility workers.
Would this require a 2 seperate transfer switch scheme? Also if the solar panels are disconnected from any load is there a Voc or high voltage with no where to go issue?
You are not understanding how a grid-interactive inverter works. The output of a grid+interactive inverter depends on utility voltage to operate. When utility voltage is lost, the inverter immediately ceases AC output. That is why you can, and should, interconnect it between the meter and the line side of the generator transfer switch. This is one of the most popular ways to interconnect solar today. See the National Electrical Code section 705.12(A) regarding supply-side interconnections for photovoltaic systems.
How the RV automatic transfer switch works. Typically, the wiring diagram of a typical RV switch shows you a three-way switch consisting of one output port and two input ports. The Motorhome switch prioritizes power coming from the generator. A logic-controlled sensor will trigger the device to draw power from one source to another. When the controller detects the generator at the optimum 120 volts, the ATS switches back to the generator input. Meanwhile, the switch disconnects from the onshore power supply.
This article is about grid-interactive solar panels. RV and off-grid inverters are multi-mode inverter/chargers that can have multiple inputs. That is a different subject entirely. Multimode inverters can be used in battery backup systems for homes, but they are also different than mobile inverters in some ways, specifically due to grounding issues.
What about the new Generac DC generator/Pwrcell/Solar system? Why can’t we retrofit that system onto an existing Generac AC generator with a AC/DC convertor?
This article focuses on connecting a generator to an existing grid-interactive solar energy system. Introducing batteries into the equation is a whole other animal. There are many ways to do that, all of which require transfer switches of some sort, often built into the inverter/charger.
The “new” Generac system is not new at all. It is equipment purchased from another failed solar energy company that was rebranded with a name brand and fancy marketing. Most of the new and innovative companies are moving to an AC Coupled system architecture for grid connected systems with battery backup. The Generac system is an older DC Coupled concept. It’s not entirely bad, but not something we would recommend.
Extremely instructive. Thanks for this. I am trying to educate myself and your answers are by far the best I have found on the web.
Your exemple is showing a traditional on-grid inverter. What happens if the inverter is an hybrid system, with a battery and ATS built in the inverter to allow to immediately switch from grid to battery energy. The whole system is equipped with zero export to prevent excess power from going back to the grid. If the connection is on the incoming line of the generator ATS, how would the generator detect that there is no incoming as energy would come from the battery / solar installation?
That’s not how a typical hybrid inverter works with a generator. You would not need a generator ATS.
In a scenario like that, as you mentioned, the hybrid inverter/charger IS the ATS. The inverter/charger would have two AC inputs, one for grid, and one for generator. When the grid input is lost, as you pointed out, the inverter would draw from the batteries and power the loads on the AC output terminals. The inverter/charger would have an internal or external generator automatic start module that starts the generator under programmable circumstances (i.e. low battery or high load). When the generator starts, another transfer occurs and the generator output is passed through directly to the loads via the AC output terminals. The generator input also provides power to the internal charger to charge the batteries. When the grid returns, the generator shuts down and the grid again powers the loads directly and the grid also provides power to the charger if necessary.
All of this switching occurs within the inverter/charger seamlessly (or multiple inverter/chargers if stacked in parallel and/or series as the design may require).
Here is an excellent example of a wiring diagram, courtesy Sol-Ark from their 12K inverter model:
I should also mention that there are many, many ways to integrate generators and inverter/chargers depending on the scenario. Here is another strategy from the same Sol-Ark inverter manual that shows a generator transfer switch ahead of the main panel, but the ATS does not start the generator when the grid is lost. That functionality is handled via a 2-wire start signal from the inverter via whatever logic is programmed in (i.e. low battery, high load, etc).
Jason, Is the fusible disconnect between the solar and transfer switch installed to isolate the solar from the working grid so work can be done to the solar in a safe manner? I’ve got 2 strings of Enphase microinverters with rapid shutdown that enter into a combiner box with breaker for the whole solar array and also individual breakers for each string allowing them to be isolated before they enter into my main panel on the load side. I’m looking at introducing a portable backup generator connection which would necessitate the installation of a transfer switch and relocation of my solar connection from inside my main panel (load side) to the line side of the transfer switch. Because I’ve got the solar combiner box, with breakers, is the fused disconnect in your drawing in the video redundant and not needed? Thanks for the great video.
Josh, no, the fusible disconnect is not redundant and yes, it is absolutely required. In some places it can be a single circuit breaker enclosure with circuit breaker, but it must be rated for use as service equipment. There are more requirements beyond that as well. No offense, but if you are asking that question, you really need to hire a qualified electrical contractor that is experienced with this. You were talking about very serious and very dangerous electrical work on the live line side of the premises main disconnect.
This probably sounds uneducated, but I am only 2 days into researching home energy production.
Let’s say I had a hybrid system set up similar to what’s being talked about here. I understand that any excess solar power could be (state pending) sold back to the grid.
Say the conditions are such that no solar is being produced. Do I understand correctly that the inverter can be programmed (pending generator capabilities) to automatically start the generator to provide power? And if so, would the generator essentially take the place of the solar power and follow the same sequence of allotment: home, batteries, grid?
Can the generator net meter? Theoretically and/or legally?
Yes, the inverter can be programmed to start a generator automatically. It depends on the hybrid inverter and how everything is designed and configured, but typically when the generator is running it will pass power directly to the loads and also power a battery charger that is integral to the inverter. In this scenario, the generator cannot backfeed the grid because the generator would only automatically start if the batteries needed to be recharged, which would only happen if there was no connection to the grid (i.e. the grid is down). You would not want to backfeed the grid anyway, from an economic standpoint, with a generator. You would be hard-pressed to produce power with a generator at a cost that is less than you would get from net metering. Regardless, you could not connect the output of the generator directly to the grid for safety reasons, but you could, in theory as you say, recharge batteries that would in turn use a UL listed grid-interactive inverter to discharge to the grid. Theory is one thing. Reality is another.
So the answer to your question about the generator taking the place of solar in the same way is, no. Solar can be connected in two ways – DC coupled or AC coupled. In a more traditional DC coupled system, DC power from the solar panels passes through a charge controller to charge the battery. In an AC coupled system, which is becoming more popular for a variety of reasons, the AC output of a grid-interactive inverter is coupled to the AC output of the hybrid inverter, which in turns converts the AC output of the solar panels to DC energy through the integral battery charger. This can occur when the grid is present because excess solar power can also be sold back to the grid. In the case of a generator, it is not AC coupled in the same way. When the generator runs, the AC connection to the grid is removed and the AC output of the generator powers the integral charger to convert AC generator power to DC battery current. The loads are disconnected from the inverter AC output because the generator passes through power directly to the loads.
I will note that there are many kinds of systems and new technologies are coming out that change this paradigm. For example, Enphase’s Encharge batteries can be connected in a way where the AC output of the generator and solar panels are both directly connected to the loads, and a microgrid interconnect device (MID) turns off individual solar panels via microinverters to prevent excess solar power from backfeeding the generator. Aside from this, there are endless configurations and combinations of equipment that can work safely and properly.
Hi Jason, just so I’m clear. I have a solar system that is tied to the grid with Enphase IQ 8 micro inverters. When the grid goes down my power goes down. I want to be able to integrate a small portable Honda EU20001 generator to my main panel to power up a couple of breakers for emergency purposes only (ie lighting). I’m planning to install a generator interlock kit. Just so I’m crystal clear, as long as my main panel disconnect is off and the breaker to my solar array is off I’m good to go to use the portable generator? There won’t be any issue with damaging my my solar array or back feeding the grid?
I would not say you are “good to go.” While this may work from a purely technical standpoint, it does not meet the requirements of the National Electrical Code, manufacturers’ instructions, and it’s not safe. It’s not safe because it’s not failsafe. A mistake can easily be made (it’s physically possible) where a user could activate the generator and forget to turn the solar breaker off, or not know to do so. The building code is there not only to protect you, but future owners of your property.
The right thing to do is what’s suggested in this article. Move the solar interconnection to the line side of the main disconnect, and thus the line side of the generator interlock.
hey Jason, is there a way to connect and charge a portable solar generator such as a Titan or Jackery to my home solar system on the roof?
In short, no. Those products are toys compared to roof-mounted solar panels. They cannot accept the output of even a few of the modern solar panels we install on roofs. And the way they connect are totally different. Let’s put it in perspective. The largest Jackery “solar generator” is only 1,534 watt-hours of capacity. A typical system we install on a home today might produce around 50,000 watt-hours of electricity. Where would all of that go? People really overestimate what these portable batteries can do. They are really just for charging cell phones and running small things like lights and fans.
For more perspective, the smallest battery system we offer for homes in Florida is over 20,000 watt-hours of capacity. And that won’t even run air conditioning. That just covers the basics like lights, fans, outlets, and refrigeration for the most part.
Hope that helps!
How exactly is the lines from the solar power disconnect connected inside the transfer switch along with the main line from the meter? Can any manual transfer switch work? Like the Reliant Controls TWB2012DR Panel/Link®? Is there one you would recommend for a integrated 200 amp meter/main combination device?
In your reply to 1st comment to the article, you said that it can get complicated with meter/main combination device. You also said some contractors may reduce the main breaker rating from 200 amps to 150s to allow larger load side connection. How can you tell this has been done? I have square D meter/main combination with meter on the bottom and the solar connection on the top right. According to my solar app, my solar power rating is 4.02 kW. What other complications are there from install a manual transfer to a meter/main combination device? Thanks.
That’s a lot of questions, and it sounds like you need a qualified licensed electrician to help you with your options. I’ll try to answer some of your questions here.
The inverter output conductors are connected to the line side of the transfer switch using any approved wiring method. This could be line side lugs with multiple terminals, Polaris taps, insulation piercing taps, or any number of ways. I don’t know about “any” manual transfer switch, but most likely the answer is yes. I don’t see why the Reliant model you mentioned couldn’t be used, but each situation is different. I can’t answer your question about your specific meter/main. If you have a 200A meter/main with a 200A bus, you may be able to dowrate the main breaker to 175A or 150A or even lower if a load calculation reveals it is code compliant. That may not be in your long-term best interest however. When you are looking at a meter/main and a generator transfer switch, things get too complicated for general answers. You have to consider every conductor and busbar, and there may be limitations of the rating of load circuit breakers within any given load center. You also need to ensure that the solar interconnection is on the line side of the transfer switch, which could be in many places within your wiring system. So again, you need a qualified person to determine what is code compliant.
I had to convert a solar install from being tied into a breaker in the main panel to being tied into the line side of the utility after installing a generator on a home with an existing solar system. I used a fused disconnect with the same amperage rating as the original breaker had. My question is about the Current transformers that feed info to the solar system main board. They were originally on the line side of the main circuit breaker in the electric panel. I assume it is to read the power coming from utility vs the power the solar system is providing. Do i need to move those current transformers to the line side before the solar/utility tie in? That would let them read the utility power before the solar ties in just like it was originally installed. Is that correct or am i mistaken?
I am going to assume you have an Enphase microinverter system based on what you are describing. The answer is yes or no. Yes, you can simply move them to the line side of the solar interconnection point. However, you may be able to solve this without moving the CTs. You might be able to simply change the setting in the Enphase Enlighten portal to “Load Only” depending on their location. You will need Enphase or an installer to do this for you (remotely).
Word of warning – you may really mess up your consumption monitoring if this is done wrong. In some cases it will result in a massive spike of the consumption number due to the way Enlighten works, so you may have bad historical data or you may even want to wipe out all of your consumption history.
Hi Jason I’m in electrical contractor in California and ran across your informative site here. I’m currently installing a 20 kW Kohler generator with 200 amp transfer switch onto a home that has existing solar panels and the grid tie inverter system. I became aware that I need to isolate the solar system from the back up generator. The homeowner bought a accessory from Kohler That he presumed would do this for us. The accessory is basically a normally closed contactor . I don’t think I will need this because I think I can isolate the solar by installing the output of the inverter on to the Line side of my utility company lugs on the transfer switch. Do I sound like I’m on the right track? Thanks for your help in advance
Never mind Jason you already answered my question in your video! Thanks again turns out I was right! Lol
Glad to hear it! A contactor is not failsafe, so moving solar to the line side is the best option.
Hi Jason, excellent articles. Glad I found this site. If I disconnect my home from the power grid, flip the solar power main cutoff handle down (off position), and throw all the solar circuit breaker panel switches into their off positions, can my generator feed the house without any back-feed from the solar?
CAN you do it? Yes. SHOULD you do it? No.
The solar output and generator need to be electrically isolated in a failsafe way so that there is no chance that they could work in parallel. In your scenario, someone unwise to the scenario could flip a breaker on and cause a catastrophic issue. You must connect it in a safe manner.
One of the best answers I have seen on the topic. Right on the money. Perhaps you could expand on it. For example, run through some scenarios of consumption levels, the types of solutions people may encounter or consider, and the pros/cons of these.
As an example, I am looking to design a system for my home. I live in a hurricane prone location (puerto Rico). Currently, I consume about 20,000 kwhs per year. Don’t have solar nor a generator. Visited with several vendors and you can imagine the range of absurdity I got back. Excessive panels, limiting microinverters, costly batteries banks that require us to basically all move inside the fridge to preserve battery life, etc. After much consideration, I am currently inclined to install 26 450-480 w panels, optimizers, string inverter (Sol-Ark or Solaredge), charge monitor, ATS, wifi connectivity module, and a 15-Kw whole-home diesel generator for backup. As I see it, until Ford or GM come up with a fast charging 150 – 200 kwh battery (V2H) SUV at a reasonable price (the ford lightning extended fleet version may be the one), the current cost to have some 60-70 kwh of battery to provide for one or two nights is prohibitive. For now, the generator with 100-150 gallons of diesel will have to do as backup in an emergency (couple of hours or Maria like 45 days).
There is no question that batteries are expensive. But not everyone has the luxury of space for a giant diesel tank or space to bury a propane tank on their property. There are different solutions for different scenarios. Sol-Ark inverters could be a very good option depending on your scenario. We do not recommend SolarEdge due to poor customer support and product issues. The Enphase battery is the most consumer friendly on the market at this time, and has excellent support. As an industry leader, Enphase is a pretty safe bet. But there is a limit to the battery capacity, which might not meet your needs. You mentioned 20,000 kWh per year. That is not the important number for backup needs. You need to identify your worst-case scenario. How much energy would you need after a storm per day in, say, September? I don’t understand your comment about having one or two nights of battery capacity. A properly sized system with solar panels can provide seamless power indefinitely. We had clients on Hurricane Ian ravaged Sanibel Island who never lost power through the storm or afterward. It comes down to the cost versus the peace of mind and convenience for each individual. In your scenario dealing with diesel fuel might be the best option. For others, the simplicity of battery backup might be better. That’s where a good, experienced, local, and honest solar contractor comes into play. They will guide you properly.
First and foremost, thank you for your time and what you do
So from all the comments above, a generator and a solar tied grid system cannot be tied electronically together unless there is a “manual” disconnect in place.
A grid tied solar system though can be switched off “from the grid” and run independently by a hybrid transfer switch and allowed to continue to systematically charge installed batteries so that the solar system is still feeding the house regardless of the grid being down.
Is this correct? I’m assuming the second part above is doable as you’ve illuded to this previously, meaning there are actual hybrid switches that perform this function? But is the first part above also doable, meaning is there an actual approved “manual” switch that can perform this task?
It’s a bit more complicated than that. No matter what, when a generator is being used, there must be physical isolation between the grid and the backup power system. This would be in the form of a manual transfer switch or an automatic transfer switch. Sometimes this is accomplished with a hybrid inverter that incorporates a failsafe transfer switch that does not allow the inverter output nor the generator output to pass through to the grid. Usually these inverters pass generator power directly to the loss and use the integrated charger to charge batteries with generator power. The inverter stays off while the generator runs (or runs in “reverse” to charge batteries rather than discharge them).
But there are other ways to do this. Modern backup systems are now using gateway devices or smart controllers of some kind. These are now known as microgrid interconnect devices. They also isolate generator and inverter output power from the grid. At least one system (Enphase) allows the generator to be AC coupled with multi-mode inverter output and prevents backfeed to the generator by curtailing inverter output when necessary. But most microgrid interconnect devices will pass generator power to the loads while inverter output charges batteries, and then turn the generator off automatically when batteries reach a programmable state of charge.
There is no single way to accomplish the goal of solar with battery backup anymore. Various systems can work together if paired with the right interconnection gear. Some system are closed and proprietary. Some systems are agnostic as to batteries and generators. There are pros and cons to each approach. Discussing your goals with an experienced solar contractor is the best way to get the system that is right for you. Don’t rely on the Internet as your sole source of information or “one-size-fits-all” approaches.
Maybe I’m a total idiot, but if you connect your inverter to the line side of your generator switching panel as suggested, won’t you defeat the safety effect of the transfer switch and run the risk of backfeeding the grid with your inverter, and possibly toasting a lineworker trying to repair the power? What am I not understanding here?
Grid interactive solar energy systems shut down immediately upon grid loss when installed correctly. The inverter senses a loss of grid and will not backfeed it until voltage is present again. This concept is called anti-islanding and as part of every inverter listed to the UL 1741 standard.
A transfer switch is not necessary for a strictly grid-interactive inverter of this type. Other hybrid (multi mode) inverters typically incorporate a transfer switch internally.
Hi Jason, Thanks so much for this knowledge. I have looked for this UL Listing on my inverter and in the Specs, with no Luck. 6000 watt split phase 24 to 220 volt Sigineer inverter/charger. Though it’s Chinese built it Seems to have all the bells and whistles. I am in rural Mexico so good tech help is difficult to come by. I have lived off grid for 40 years since the first Solar panels were available, but never before with Generator 14kw Kholer propane with auto transfer switch. Though grid connected it is not a system to run meter backward as no credits are given. just covering emergency outages post Hurricane etc. Is there a possibility that you or anyone at your business would be willing to do paid consulting for the more complicated aspects of my installation?
That is not a grid-interactive inverter and cannot be connected to the grid in any way. You need an external transfer switch mechanism, whether manual or automatic, it doesn’t matter. The output of this inverter needs to be completely isolated from the grid.
We do not do this type of consulting, and we only work in our local service territory. You should find someone local that is knowledgeable to assist you.
Hi Jason! I have: 1) a solar roof (capped at 9,600W with 4 disconnect switches at various locations in the solar electrical system) with: 2) a 10,000W battery pack, and; 3) a bi-directional pole meter, all professionally installed. I’m now considering installing “Generlink”, which is a legal, automated transfer switch (equipped with a plug-in to accommodate the cable of a gas-powered generator under 10,000W) and that Generlink fits into the pole meter (legally), and it prevents back feeding to the grid. – So during a grid-down scenario, the battery pack kicks in automatically to supply a secondary electrical box with a limited number of critical house appliances but, when the battery pack is depleted, I’d start the gas-powered generator via Generlink (which is located on the pole meter) to supply selected appliances on both the main and secondary electrical boxes. Would that work? What are the considerations? Do I need to have another transfer switch installed and, if so, where? [By the way, thanks for what you do: it’s much appreciated.]
That’s not enough information for me to answer the question. I would need a whole lot more detail about your existing equipment and how it is connected. However, the answer is probably no, you can’t do that. A Generlink is generally incompatible with a solar energy system with battery backup. It’s not the best way to accomplish it anyway. The right answer depends on what you already have, but you can probably take Generlink off your list of options. You might be able to make it work with additional switchgear in either an automatic or manual way, but it certainly would not be the most elegant solution. You need a local solar pro to help you integrate a generator properly.
A pleasant day sir.
I am currently working in a resort here in Maldives. I am planning to propose a solar power for the resort. Our daily average power is 1200 kilowatts and of course it could run up to 1500 kilowatts during peak season. we have four 1650KVA generators and we run them according to our needs daily. As per calculation for the available space for solar panels, we can produce around 210 kilowatts of solar power. my concern is if we can parallel the solar power with our generator. On what I have read from your article, that solar panel will back feed the generator and may inflict serious damage to it. Considering our power consumption and power house set up can we run the solar power together with our generator?
Your question goes well beyond the scope of this article and the ability to answer your question succinctly. You will need some kind of multimode inverter setup with Generator input to make this happen. You need a design professional that works in your area of the world to assist.
Interesting article. I have Enphase IQ7+ microinverters backfeeding my panel at 240 volts with a 20 A breaker in the bottom right. Two questions…
1) What if a generator is connected via a load side transfer switch? My understanding is a load side transfer switch never energizes the bus bar/house panel thus never fooling the microinverters into producing with solar.
2) If an inverter hooked up to a transfer switch or interlock outputs only 120 VAC, it would not be able to active a grid-tied PV system since those look for 240 VAC. Right? In this case it would be fine to use a standard interlock or transfer switch with the solar breaker left in the main panel.
Yes, you can connect a load-side transfer switch because the solar will be connected on the line side of the transfer switch.
I’m not sure what you are saying about the alternative 120V source, but presumably, you could connect 120V of a generator or inverter output in parallel with one leg of the microinverter circuit without issue. I’m not sure why you would want to do that given that it would only power half of the loads in your panel (one leg). If you tried to power both legs with the same 120V source you could creat a serious safety hazard (i.e. overloaded neutral on a multiwore branch circuit). What I’m saying is – Do not do this. While technically it would not allow the microinverters to produce power, there are a myriad of other reasons not to do it.
Good advice. I just wanted to confirm that this article (as you said) is for a whole-home generator install. If someone is installing a load-side transfer switch and only backing up six or so circuits (critical loads) they are fine to leave the solar breaker backfeeding in the main panel. Solar would be isolated from the generator. Correct?
You are correct.
Hi Jason thanks a lot for all this Q&A!
I’m wondering if a Smart Management Module like the Generac 7000 might work for this.
It supposedly will automatically disconnect a 240V load (in this case the solar panel inverter output) if a generator is detected to be running, and then switch back on when the generator turns off. I believe the sequence would be:
1/ Line voltage goes down
2/ Solar inverters shut down
3/ Generator kicks on about 15 seconds later
4/ SMM opens, disconnecting solar inverters
5/ Solar inverters stay off
6/ Line voltage comes back up
7/ Generator shuts down
8/ SMM closes, solar inverters come back on
Let me know what you think… thanks!
No, that will not work. Aside from not being a failsafe solution, that particular module does not work that way. Even in Lockout mode, if the generator has enough capacity (it is not overloaded) the solar inverters would receive power (voltage) and remain connected to the generator output. That module is a load shedding device that prioritizes what can stay connected to the generator based on available capacity. The Lockout Mode is for loads that are non-critical and don’t need to be powered on generator power in an off-grid event. However, it is basically like the lowest priority, but will still be powered if the total load is low.
From the manual:
NOTE: LOCKOUT mode works in an overload condition,
which is determined by generator frequency. If generator
has sufficient capacity, the load will be allowed to turn on
even if control is set to LOCKOUT mode.
There are plenty of off-the-shelf and custom controls that can shed a load in a utility outage using a contactor, but this is not a failsafe way to interconnect solar inverters in my opinion. The safe way to accomplish this is to put PV inverter output on the line side of a generator transfer switch, like this article and video suggest.
Note that the Enphase IQ microinverter systems can be paralleled to the output of a generator when incorporating their System Controller, which is a transfer switch and microgrid interconnect device. The System Controller senses backfeed from the inverters to the generator and throttles PV output accordingly. Enphase does allow you to use their older M-Series microinverters in this manner to parallel IQ batteries with a generator, but they require shedding the PV with a contactor, which again, in my opinion, is not a failsafe solution. Others may disagree. Since the manufacturer has a UL listed (tested) solution, I would be comfortable enough to do this, but not without hesitation.
Everyone seems to want to get around the issue of relocating the solar interconnection point when installing a generator with an existing grid-interactive solar energy system. I don’t see why. It’s easier to just do the safe thing instead of trying to reinvent the wheel and introducing additional potential failure points in a less safe manner. The cost of a 60A or 100A disconnect switch with overcurrent protection is very low and it’s easy to accomplish. You can use a service rated fusible safety switch or even a circuit breaker enclosure. You may need to use a circuit breaker with a high interrupt rating. The Square D QO enclosures can accept a QO2##VH breaker which is 22kAIC. Again, there are lots of cost effective ways to do this without a less safe and failure prone load shedding contactor solution.
Schneider XW Pro inverters come with both grid (AC 1) and generator inputs (AC 2). These will work fine with a DC coupled system. However, if you are building an AC coupled system where you are adding back up capability to an existing grid tied PV inverter, you have an issue. There are three basic solutions.
1. Make sure your solar never runs when the grid is down. This would mean tying your solar in on the grid side of the inverter (AC 1). The inverter will insure that only one source is selected at a time. In this case you could not use solar during an outage to charge your batteries.
2. If you tie your solar into the AC output as is recommended for an AC coupled system then you have to insure that your generator and your PV inverter can not run at the same time. Schneider recommends using a contactor for this purpose. A contactor is simply a multi-pole switch that can be turned on or off by a signal wire. For this purpose you would use the 2 normally closed terminals for the solar and 2 normally open contacts for the generator. Because mechanically it is single switch, it is safe. If it fails to energize, you just won’t have generator power. Then use the generator voltage as the signal. When the contactor is energized it will open the solar path and close the generator path. If you are setting up your system to auto start your generator when the batteries get low, this is what needs to be done.
3. Use a manual transfer switch so that only solar or generator to be selected at the same time.
The frequency shifting that is used to control the PV inverters is not instantaneous. It has defined ramp up and ramp down rates. In a grid down situation It requires time to balance the loads with the PV production. So while that is happening excess PV must be pushed to the batteries. This all requires that your inverter and batteries must all be sized large enough to accomplish this. Schneider also recommends that a fail safe “battery overvoltage disconnect” for the PV be installed. They provide a 12 volt signal that can be programmed for this. So add in a couple of control relays to you contactor. You can then insure that the PV is automatically disconnected if the voltage goes to high or the generator is turned on.
Designing a DC coupled system is relatively simple and straight forward. AC coupled systems where you want to be able to charge your batteries from either solar or generator power during a grid down situation can get very complex. That is why DC coupled would almost always be the preferred solution for an off grid system. So how often will you really experience grid down situations and for how long?
Thank you for your comments, and I agree with what you wrote, but this is not really germane to the topic here. We are not talking about battery backup systems. We are talking about what happens when you want to add a generator to a simple grid-interactive PV system (non-backup).
I will make one comment that is very important with respect to this article. I know that inverter manufacturers may suggest using a contactor to shed a PV input when the generator is running. But the method you recommend in #2 isn’t so simple. Because the solar is connected to a normally closed contactor, it requires a mechanically interlocked contactor, which would be very expensive for this type of application. The coil must be energized to open the PV circuit and close the generator circuit. Contactor coils fail. A control wire could become loose. In short, this is not a failsafe solution and does not meet my standards. It could result in the generator failing to operate at a critical time during a grid outage.
Anyway, this would be very expensive and pretty impractical for a 100A (or larger) generator output. The cost would not make sense compared to simply moving the solar interconnection point. Basically, you would be introducing a second transfer switch into the system – hardly a practical solution for integrating a generator with a non-backup grid-interaactive PV system.
The purpose of writing this article was for me to convey to generator companies how they can deal with typical residential grid-tied PV systems when installing a typical 200A transfer switch used for whole house generators, which is very common in this area of the world.
Thanks again for your contribution.
I would agree that on simple grid tied system moving the interconnection point to the line side of the transfer switch would be the way to go. Simple, fool proof, and cheap. This was essentially option 1 in my original post. Most people don’t realize that during a power failure their grid tied inverter becomes useless. They think I already have solar power so why can’t I use it during an outage? Adding a generator does keep you from losing power in an outage, but it still won’t let you run your solar system. Of course if you factor in the value of that solar power production you are losing vs the cost of adding a full battery backed up hybrid inverter to your grid tied PV, it is hard to justify the cost of this solution.
For me at least the decision to go with a full house battery back up system was more a matter of not wanting to be reliant on the grid, which I believe is become more and more unreliable. I built a system that provides layers of protection. If I lose power, I have batteries. If I have sun, I can recharge the batteries from solar. If I don’t have sun I can use a manually operated generator to charge the batteries. The thing is getting this all to work together is a complicated problem. Because I have a relatively large PV array, I actually needed 2 XW Pro inverters to be able to handle the power in a grid down situation
When adding a generator to an AC coupled system Schneider recommends using a four pole contactor with 2 NO and 2 NC poles. It is a single contactor so it is inherently interlocked. These contactors cost about $100.00. When energized the solar disconnects and the generator connects. Note that unlike a transfer switch, where two sets of inputs feed the same set of outputs, there are four completely separate paths through the contactor. The generator connects to L1 and L2 of AC 2, while the PV connects to the L1 and L2 of AC out. Inside the inverter when AC 2 becomes the qualified source, a relay closes which would then join these two sources together causing the issue. When using an AC source the inverter essentially becomes a pass through which will allow the PV to back feed the generator. US breaker makes these contactors in various sizes going up to 95 amps.
It just may surprise people that in a battery backed up system with a hybrid inverter, you still have an issue with back feed when your system is AC coupled. The inverter does nothing to prevent it. So you can’t run PV and a generator at the same time. In this case the generator is there as a supplemental source for cloudy winter days, when solar power is insufficient. I have a 12.5 KVA solar system and a 30 amp generator connection, so for me at least it is the PV that requires a 65 amp contactor. Running a mid sized (8000-15000 watts) generator 2-3 hours a day is enough to fully recharge the batteries. You don’t want to run a generator full blast for hours. Setting the system to charge at 5000-6000 watts is fine for a 30 amp connection and won’t stress the batteries or the generator. My typical load is 500-600 watts in the winter time, so it’s much more efficient to do this than to run a generator 24*7 during a longer outage.
You might find a contactor for $100 for a smaller generator/PV system, but when you get into the 100A+ range, it’s going to be substantially more expensive, especially considering the enclosure, conduit, and wiring required. And you still have the issue with how to drive the coil, the energy used by the coil itself, and the fact that it can fail, rendering the generator useless until replaced. In this case, it’s not failsafe in the respect that the generator may not be usable when it is most needed. I’m not in love with this solution. But it can work safely if implemented properly.
What you are saying about AC coupled PV systems is correct for the most part, but that is what makes the Enphase IQ battery system unique. Its Smart Controller can allow you to run a generator AC output in parallel with the PV AC output and battery AC output simultaneously. In all honesty, aside from simplifying the integration in some ways, I don’t see a huge benefit in terms of fuel reduction, but the system is smart enough to only run the generator for certain circumstances. For example, start the generator when batteries drop below 30% and stop the generator when they reach 70%. The Enphase system is one way that you can integrate a generator with an AC coupled PV system architecture.
The other option of course is a traditional DC coupled system. But there are downsides to that as well, notably the fact that generator power simply passes through to loads and solar can only charge batteries while the generator runs. You also still need a rapid shutdown system in most cases for he PV, reducing the cost benefit of a DC coupled system and increasing complexity.
There are pros and cons to all of the possible scenarios. Simple and servieceable is best in my opinion.
Again… this is all way beyond the scope and purpose of this article, but thanks for your contribution to the discussion.
Yes, Enphase does a lot of proprietary magic in their system. Just like their “Daylight” backup option that can provide limited backup without a battery, they take advantage of the ability of their IQ8 inverters to respond very quickly to changes in load. They can adjust the battery storage and the inverter output to make sure inverter back feed to the generator does not occur. It is essentially like an off grid version of “Zero Sell” where you setup a system to not export power back to the grid. Enphase has apparently made this all work well enough that they can let the PV and Generator work in Parallel.
With other companies “Zero Sell” works well enough to keep the utility company happy when you don’t have an interconnection agreement that allows export (although some leakage may occur), Schneider, apparently, is not confident they can manage the power well enough to not create problems between the PV inverter and the generator. Since I would only run this when it’s cloudy, I’m not concerned about being able to parallel PV and generator.
As far as wiring this up, there is actually room in the PDP panel to add the contactor in. All that is required is a couple extra feet of wire. The contactor has a 240 volt AC coil so the generator AC is used to close the generator contacts and open the PV contacts. While anything can potentially fail, these contactors are rated for 1 million cycles. If you turn your generator on and off 10 times a day, these contactors should still last 270 years. They are used in industrial processes everyday to turn large 480 volt motors on and off hundreds or thousands of times a day. Look in any multi function inverter and it is already full of relays. Generators on the other hand are NOT particularly reliable. The useable life of a typical air cooled LPG or Natural gas generator is 500-1000 hours. Run your generator for a week after a hurricane and it is 1/3 used up. Given these generators cost $10,000 that is $3300 plus fuel. You might be better off bailing for a week and staying at the Ritz Carlton.
I agree about Enphase having a great solution. There are many things about their system that I love including their redundancy and reliability. It’s why I went with their micro inverters. They have been rock solid and I have had zero issues with them. The Enphase batteries have zero moving parts and use 12 micro inverters. Lose 1 and you still have 11 that are working. If I could afford an Enphase storage system, I would have bought it. 30KWH of batteries with 10KW of inverter power and the controller are a minimum of $35,000 just for the hardware. Enphase prides themselves on being DIY unfriendly, so add installation and it’s more like $50,000 to $60,000. I was able to get 30KWH of storage with 24KW of peak inverter power for about $20,000. These Schneider inverters are built like a tank. They will easily power air conditioners and large tools like saws and welders. Still if I had 40 grand lying around I didn’t need, I would have gone with the Enphase. I think I could get by without welding during a power outage.
I would NOT recommend an AC coupled solar install as a DIY project for most people, but I have done a lot of electrical work. I also have an engineering background. I actually really like doing this work and I love my system.
We charge nowhere near that for Enphase batteries. And while you might have got what you did for $20k, that is just the backup component and does not include the charge controllers on the DC side which you would need to add for a more apples-to-apples comparison. But regardless, it’s hard to compare AC and DC coupled solutions. They are different animals with different strengths and weaknesses. You are better off with an AC coupled solution if you are going to spend 99.9% of the time on-grid, which is what most people do. DC Coupled is the only way to go for true off-grid, although there are arguments for hybrid systems with both in some scenarios. The Enphase system is truly unique in its generator integration, and yes is commands a higher price than other solutions, but it fits the needs of the majority of the public best in our opinion. Consumer friendliness and serviceability are kind of big deals in the retail solar energy business.
As for DIY friendly, it’s becoming harder to find ways to make that work. Between required manufacturer certifications, safety concerns, and code complexities, there are many reasons to let us professionals do what we do. Add to that the fact that we get most jobs done in 1-2 days, you have to ask yourself if you are prepared for weeks of headaches if you DIY.
Generac transfer switches have a dry contact that switches with the transfer and can be used to disable an inverter if your inverter has a port to do that. This would be all you need to safely connect a solar system downstream of the transfer switch and a generator. The relay method would also work. I would wire it so the relay is always closed (on) so in the event of a failure it would open and disable.
A couple of notes. Selling power to yourself vs. wholesale to the power company saves you money as long as you use all the solar you generate. For smaller systems, this is probably the case. Modern meters probably won’t run backwards so doing it this way if you generate more energy than you use you will be giving it away back to the grid.
This is probably not technically legal in most areas. I would not advertise you are doing it. Just safely do it! Use a double breaker of in your panel rated at just over the maximum your panels and inverter could produce.
Thank you for your contribution, but the information you provided is not applicable, not advisable, and/or inaccurate. Here is why:
Transfer switches may have dry contacts that open when grid power is lost, but 99%+ of the grid-interactive inverters installed today in the US do not have inputs for this type of control signal. The reason is twofold. First, it’s not necessary since they are inherently interactive and require a grid voltage to operate. It would be redundant to provide this control mechanism. Second, most inverters installed today are microinverters. They are on the roof and they are numerous. It would be impractical to provide a control signal to each.
You suggest using a normally open relay (you call this always closed, which is inaccurate). This requires grid voltage (from the line side of the inverter) to power the relay, which closes it. However, when the generator starts, voltage returns on the line side of the inverter, closing the relay again. How would you propose using a normally open relay in this case? It would have to be upstream of the transfer switch with proper overcurrent protection. This is impractical and it is prone to failure, which could cause the inverters to cease functioning if the coil fails. It’s just not a good idea, and probably more expensive than just interconnecting the inverter the right way (on the line side of the transfer switch).
Grid-interactive systems imply that the customer has net metering and a bi-directional meter. These meters spin both ways and record energy in both directions. Some areas use two meters to do this and do net billing. For some meters that are not bi-directional (i.e. the customer does not have a valid interconnection agreement) the meter may calculate the absolute value of current flow, so you would actually be paying the utility company when you send them excess power. It’s important to work with a solar contractor, preferably local, that knows what they are doing.
I don’t know where you are getting that “this is probably not technically legal in most areas.” Interconnecting a grid-interactive inverter is most certainly legal in most places in the US. Some places have export restrictions (no export or export limits). Modern inverters handle this with ease.
Using a “double breaker” (a two-pole breaker is what you presumably mean) “just over the maximum” is not accurate. A properly designed system uses overcurrent protection in accordance with the National Electrical Code version in effect in each jurisdiction. The proper overcurrent device rating is calculated using the methodology in NEC Article 690 by taking the inverter output rating, or combined inverter output rating as the case may be, and multiplying it by 1.25 and using a breaker that has a minimum rating higher than that. It could be much higher than that provided that the conductors are suitably rated. In addition to this basic rule, the manufacturer’s instructions must be considered.
This article is intended for electrical professionals, particularly in our service area of Southwest Florida to learn how to properly address existing solar energy systems when adding a generator. This discussion has gone well outside of that intent. But it’s important to be accurate. Electrical design and installation isn’t something you should learn from the Internet, and you should always consider the source of your information. There is a lot of misinformation out there about solar energy and a lot of DIY’ers providing opinions that are inaccurate or outright dangerous.
Thanks for being a resource on this issue. We have a situation where the customer has a house and a barn. The main electrical service comes into the house to a 200A main brkr panel. As far as I can tell, the house service feeds the 200A main brkr panel in the barn. They have a standby generator with an automatic transfer switch. The barn is the place for the solar. Given all the landscaping, trenching between the barn and house so that we can interconnect the solar on the line side of the transfer switch might not be possible. Do you know of any other device, perhaps wireless or radio, that can make sure the grid tied inverter stays off when the grid is down even when the generator is on? Or any other thoughts?
That’s always a tough situation. You’re looking at tranching the solar conductors back to the line side of the transfer switch. It’s not ideal, but it’s the right thing to do. While there are ways to use wireless relays to make it work without trenching, it’s not going to be failsafe, or it’s going to be unreliable. I would not consider it.
If I use a manual device to require the main panel breaker to be in the off position before the breaker connecting the natural gas powered generator can be turned on and I turn off the breakers connecting two sets of solar panels to the main panel before powering up the generator is this going to be safe for the micro inverters on the panels. I will also have to shut off all 230 volt breakers on the main panel except the 230 breaker serving the well pump because I am only looking at getting a generator rated for 7000 watts running on natural gas.
No, it’s not safe. That’s the essence of this article/video. The inverter output needs to be installed on the other side (the line side) of the interlock device.
People seem to want to get around this point by relying on the manual opening of the solar breakers. It’s simply not safe because someone can make a mistake or unknowingly turn a breaker back on. It’s not a failsafe solution. Don’t do it.
We have a 13KV Sunpower Equinox grid tied system installed professionally. Our panels have microinverters and therefore we have a separate electrical panel that outputs AC and directly connects to the utility company primaries. Our house came with a whole house generator and has a transfer switch to operate about 1/3 of the circuits in the house. I understand what you are saying about the back feed problem when using a generator wired in parallel with the panels. The contractor wired the solar so it is completely isolated from the generator transfer switch. My question is if you have a battery backup system connected in parallel with the solar panels is it then safe to have the generator connected to them? The batteries (assuming a large enough amount) would consume the possibly unused generated power.
Another question if you only had solar and a battery backup system and you disconnect the solar from the utility grid is there a way to have the battery system make the solar panels able to generate power? So if the power goes out you are not only running off of the batteries.
What you are describing is absolutely possible. You just need an additional multi-mode inverter with an integrated transfer switch. This is pretty standard stuff. In your case you would install what is known as an AC Coupled system. There are a variety of ways to integrate what you have with batteries.
Unfortunately, the Sunpower Equinox system, which uses Enphase microinverters, is not compatible with the Enphase battery system to my knowledge. That is a shame, and a big reason not to go with SunPower, which has long been a closed-off, proprietary brand that doesn’t like to play nice with others. So you would need to go with a traditional multi-mode inverter system. A Sol-Ark 12k or 15k might work in your scenario. Find a local contractor that knows what they are doing and it should be pretty straightforward.
From what I understand, if you have a Sunpower SPWR PVS6 communication gateway, it will not work with the Enpahse Ensemble battery system. You need an Enphase Gateway for it to work.