Electrical schematic for solar and generator integration.

How To Install A Generator With Existing Solar Panels

In Education, Solar Design 35 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.

  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.

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