Electrical schematic for solar and generator integration.

How To Install A Generator With Existing Solar Panels

In Education, Solar Design 71 Comments

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.

 

Comments

  1. 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?

    Thanks…

    1. Author

      Hi Jerry,

      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.

  2. Can you update your natural gas generator to solar or do you need to get a whole new system

    1. Author

      Hi Penny,

      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.

  3. Question.
    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.

    1. Author

      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.

  4. 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

    1. Author

      Hi Jason,

      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.

  5. 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?

    1. Author

      Hi Mark,

      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.

  6. Hi Jason,
    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.
    Thanks,
    Andrew

    1. Author

      Hi Andrew,

      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.

      Jason

  7. 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?

    1. Author

      Hi Colin,

      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.

      1. Hi
        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.

        1. Author

          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.

  8. 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.

    1. Author

      Yeah, that’s why I have the strong disclaimer at the beginning. You would not believe how many calls I get on this.

  9. 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.

  10. 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.

    1. Author

      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.

  11. 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?
    Thanks

  12. Hi Jason.
    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?
    Thanks

    1. Author

      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.

  13. 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?

    1. Author

      Hi Chris,

      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.

  14. Hi Jason,
    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?
    Thank you!

    1. Author

      Steve,

      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.

  15. 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!

  16. 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.

    1. Author

      Hi Steven,

      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.

  17. Hi Jason,
    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?
    Grateful,
    John

    1. Author

      John,
      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.

  18. Jason

    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?

    1. Author

      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.

  19. 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?

    1. Author

      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.

  20. 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.

    1. Author

      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.

      1. 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.

  21. 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

    1. Author

      Hi Tim,

      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.

  22. Jason,

    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?

    1. Author

      Michael,

      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.

      Jason

  23. Hello Jason,

    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?

  24. 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?

  25. Hi Jason,

    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.

  26. 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?

  27. Hello Jason,
    Phenomenal information here by the way.
    Much appreciated.
    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?

    Thanks.
    Chris

    1. Author

      Chris,

      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!

  28. Hi, Jason–
    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?

  29. 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.

    1. Author

      Hi David,

      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.

  30. Hello Jason,
    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.

    1. Author

      Hi Peter,

      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.

      Jason

  31. 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

  32. 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.

    1. Author

      Hi Tom,

      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.

  33. Hello,
    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.

    Thank you.

    1. Author

      Hi John,

      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.

      Jason

  34. 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

    1. Author

      Hi Marlin,

      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.

      Jason

  35. Hello,

    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.

    1. Author

      “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.

  36. Good day Jason. Thank you for your kind expert advice. Did I miss out the reply from …
    AUGUST 27, 2021REPLY Jonathan Godfrey
    Hello Jason,

    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.

    1. Author

      Hi Rogelio,

      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.

      Jason

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