Electrical schematic for solar and generator integration.

How To Install A Generator With Existing Solar Panels

In Education, Solar Design 17 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.

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