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Adding a backup generator to my grid tied system

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  • Adding a backup generator to my grid tied system

    Hello, I'm new here, looking for advice on a small project I'd like to either do myself, or hire someone to do, depending on what I learn. I have a 12 year old 2.85 kWp grid tied system, consisting of 15 Evergreen 195W panels and and Sunny Boy 4000US inverter. It's been great, no problems all these years. Our utility company has filed for bankruptcy after their infrastructure caused too many devastating fires, so now they have begun to shut off power during high risk conditions.

    After exploring the options, I decided that for our family, rather than investing in major upgrades to our entire system, using our 2013 Prius as a simple generator will be enough to get us through these outages with minimal noise and without having to maintain a dedicated generator and store gas, (other than what we're already storing in the car's gas tank, obviously). Today I installed wiring on the Prius and tested it with our new inverter. So far, so good.

    Rather than running extension cables from the car all over the house, I would really like to wire a simple transfer switch or, better yet, an interlock breaker to my main panel, as that seems cleaner all the way around. I'm concerned that the power coming in from the generator will be somehow incompatible with the solar panels or inverter though, and like I said, they've been working so well all these years, I would hate to muck that up as a result of taking on this project without proper knowledge of the consequences.

    So, my question is, if I follow the instructions for installing a transfer switch to my home circuits, obviously I would need to disconnect power coming from the inverter during installation, but would I just ignore the fact that there is a grid tied solar system when enabling generator power to flow to the panel during a power outage? Do I need to modify the way my solar system is wired? Do I need to shut off the backfeed breaker before turning on power from the generator? Is it safe, or necessary, to shut off the main disconnect switch between the panels and the inverter during a power outage? Is my Sunny Boy 4000US able to recognize that the power from my car inverter is not the grid, and work as expected?

    Right now, my system wiring goes: Panels > Main Disconnect Switch > Inverter > Main panel backfeed breaker switch > Meter > Service pole.

    Thanks in advance for reading this. Any thoughts or advice you might have are greatly appreciated.


  • #2
    The Sunny Boy, unlike the Sunny Island, is a purely grid-interactive inverter. However it may be possible for some generator/inverters to provide stable enough to allow the Sunny Boy to qualify it as grid power.
    If that happens, there will be serious problems whenever the panel output, through the Sunny Boys, is greater than you local load. The Sunny Boy will only throttle back its output in response to a frequency shift from backup source (by design a Sunny Island). Without this throttling the SIs will attempt to force power back into the generator, with several possible consequences:
    1. The generator output voltage will rise, until the SI finally shuts off from overvoltage. Loads may or may not be damaged by the temporary overvoltage.
    2. The voltage regulator of the generator (or battery inverter) will malfunction with possible catastrophic consequences.

    The possibility of the second outcome is why the standard configuration is to always connect the PV on the grid side of the transfer switch. With a line side tap (your suggested setup) this is easy. With a load side tap and a breaker interlock instead of a transfer switch it is not possible to automate. Instead you have to also make sure you open the PV backfeed breaker before firing up the alternative power source.
    This is complicated by other conditions, such as POCO not allowing line side taps or your service equipment being a meter-main combination panel with no way to connect between Service Disconnect and the Meter. The really serious danger in your proposal is that with both the generator/inverter and the backfed PV located line side there is no way to provide a positive interlock or transfer switch to protect the grid from backfeed.

    It is, as you have seen, particularly hard to combine backup power and grid-interactive PV without spending some money on a proper transfer switch (installed in your case immediately to the right of your Service Disconnect.) That will make the Transfer Switch your new Service Disconnect and your main breaker panel is now a subpanel requiring changes to the current ground to neutral connection in that panel. If you go this route I strongly recommend that you involve a licensed electrician who is familiar with PV.
    SunnyBoy 3000 US, 18 BP Solar 175B panels.


    • #3
      Thanks for the detailed information and advice, @inetdog. Definitely time to call in a pro.

      I guess I'll be using extension cords to power my fridge, internet and various device/laptop chargers at least until I can locate and schedule this work with a qualified electrician familiar with PV.

      I spoke with a PV installer representative on the phone recently who suggested that maybe an inverter upgrade to this product called a "dcbel" (when it ships), might work in my situation. I don't have an electric car yet*, so it seems like a pretty expensive way to solve the problem, and it might not even support using my low budget Prius+inverter as a power input source, anyhow.

      I wonder if anyone makes a solar inverter that is designed to accept standard generator input as an alternative source of power. If I could just swap out my Sunny Boy for a product that did what I need it would be ideal, though probably expensive.

      *In the past, when I had an electrician evaluate my house for adding an EV charging station, the project wound up being prohibitively expensive - requiring a service upgrade, which in turn would require relocating my main panel and meter to avoid the tree in my front yard, which in turn would require installing bollards in my driveway to protect the relocated equipment and re-running all of my circuits, etc. We decided to drive less and just stick with the Prius, which is paid for.


      • #4
        An inverter which can work in either grid-interactive or standalone mode is called a hybrid inverter. Pretty much any of them would allow you to add in a generator which is feeding only a battery charger, That way there is no possibility of harmful interactions between generator and inverter,
        There are a couple of manufacturers of hybrid inverters that feature "generator support". They have the switching and control systems to allow the standalone inverter to synchronize with a generator and allow both the generator and the hybrid inverter to supply power to your loads at the same time, with either the generator or the PV being the priority source.
        You can find some information about these systems by using the Google search: "generator support"
        (That generally works better than the built-in search function of the Forum software.)
        SunnyBoy 3000 US, 18 BP Solar 175B panels.