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Using Enphase grid-tied, want to add small battery backup

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  • Using Enphase grid-tied, want to add small battery backup

    I've currently got 25 panels (@270 watts/each) with an Enphase micro inverter behind each panel.

    I've also got a "portable generator" that includes a 4000 watt true sine wave inverter/charger with about 200 AH of batteries behind it. During normal day-to-day operation, I keep this unit plugged in and it keeps my batteries charged up. If the power goes out, I can attach my portable generator to a special circuit that keeps a small critical load panel running for a few hours.

    What I'm curious about is this: if I were to leave my PV array connected during an outage, would the power produced from my portable generator create a local grid that would keep my PV array active? If so, could I then plugin my inverter/charger to my panel and simultaneously charge my batteries from the power coming from the PV array? is there any danger in doing this?

    Last edited by rscott78; 09-08-2016, 11:35 AM.

  • #2
    Short answer. No. A portable generator of that size will never be able to mimic the parameters that a power grid generates to allow those micro inverters to work.

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    • #3
      Originally posted by rscott78 View Post
      What I'm curious about is this: if I were to leave my PV array connected during an outage, would the power produced from my portable generator create a local grid that would keep my PV array active?
      No. This question comes up very regularly, and the short answer is that you can't do it. (BTW your PV array is always connected, even during a blackout - it just shuts down when it detects loss of grid power.)

      The longer answer is that there is a method called AC coupling that allows such a local grid to operate, but you are tens of thousands of dollars away from a system that can safely accomplish that.

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      • #4
        I'm curious about this. I've had this idea in the back of my mind that during an expended outage one could utilize their solar panels by "powering" them with a generator or inverter. Why won't this work?

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        • #5
          Originally posted by Graycenphil View Post
          I'm curious about this. I've had this idea in the back of my mind that during an expended outage one could utilize their solar panels by "powering" them with a generator or inverter. Why won't this work?

          IF the generator is stable enough for long enough (unlikely) to get the grid tie solar started, then the fun begins. Grid Tie solar does not have a throttle, it converts all power available to AC and crams it on the wire. The thing at the other end of the wire is the generator which is incapable of accepting the power.....

          To get this to work you need a bimodal inverter of greater size and capacity than the grid tie inverter, and a large enough battery, all configured for AC coupling. Getting this all set up will be about 1/3 the cost of the original solar PV system.
          OutBack FP1 w/ CS6P-250P http://bit.ly/1Sg5VNH

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          • #6
            Thanks ButchDeal. Theoretically, if you were using more power than the solar array was making, would it work then? Would the power be going into your home, instead of into the "grid" (generator)?

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            • #7
              Originally posted by Graycenphil View Post
              Thanks ButchDeal. Theoretically, if you were using more power than the solar array was making, would it work then? Would the power be going into your home, instead of into the "grid" (generator)?

              it would only work if you disconnected from the grid.
              theoretically it would never work as the grid tie inverter should never sync to a generator.

              In reality it might sync and when it does the generator is going to stop working.

              If you have a generator, the best thing to do is disconect the solar AND grid at the same time, and just use the generator.

              OutBack FP1 w/ CS6P-250P http://bit.ly/1Sg5VNH

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              • #8
                Originally posted by ButchDeal View Post


                IF the generator is stable enough for long enough (unlikely) to get the grid tie solar started, then the fun begins. Grid Tie solar does not have a throttle, it converts all power available to AC and crams it on the wire. The thing at the other end of the wire is the generator which is incapable of accepting the power.....

                To get this to work you need a bimodal inverter of greater size and capacity than the grid tie inverter, and a large enough battery, all configured for AC coupling. Getting this all set up will be about 1/3 the cost of the original solar PV system.

                Could you offer some suggestions on the equipment needed for it or point us in the right direction?

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                • #9
                  Originally posted by rscott78 View Post
                  Could you offer some suggestions on the equipment needed for it or point us in the right direction?
                  In order to get it to work you will need an inverter system larger than the grid tie. If you have micros though you could just use a sub set of them to make an AC coupled battery backup. Your bimodal inverter (not off grid inverter that it sounds like you have) needs to be capable of controlling the micros.
                  So you are looking at a Radian 8kw inverter, and likely 600ah 48V battery bank.

                  http://www.outbackpower.com/download...te_acc_new.pdf
                  http://www2.enphase.com/global/files...ed-Systems.pdf
                  http://www.outbackpower.com/download...te_acc_new.pdf
                  Last edited by ButchDeal; 09-12-2016, 02:01 PM.
                  OutBack FP1 w/ CS6P-250P http://bit.ly/1Sg5VNH

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                  • #10
                    Why wouldn't it sync to the generator or inverter? Wouldn't the grid tie inverter just see 240 volts/60 hz and think it is on the grid again?

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                    • ButchDeal
                      ButchDeal commented
                      Editing a comment
                      not stable enough. You need a bimodal large enough with large enough battery, and surge capability to maintain consistent and stable power.

                  • #11
                    Originally posted by Graycenphil View Post
                    Why wouldn't it sync to the generator or inverter? Wouldn't the grid tie inverter just see 240 volts/60 hz and think it is on the grid again?
                    To prevent "islanding" (a condition in which a GTI stays running even when the grid goes away because the local load is closely matched to the inverter output and the reactive impedance of that load is just right), a UL listed GTI will use several methods to make sure that the grid is there.
                    The first is to impose a strict frequency and voltage stability requirement which many generators cannot meet.
                    From there other measures include calculating the apparent impedance of the grid. That should be near zero and that behavior cannot be matched by a generator or inverter of the same scale as the local load.
                    Hybrid systems that are designed to work with generators will explicitly relax those requirements on the generator AC input compared to the grid AC input.
                    SunnyBoy 3000 US, 18 BP Solar 175B panels.

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                    • #12
                      Originally posted by Graycenphil View Post
                      Why wouldn't it sync to the generator or inverter? Wouldn't the grid tie inverter just see 240 volts/60 hz and think it is on the grid again?
                      A home generator does not produce the proper electrical properties to simulate a "grid" for a standard grid tie inverter to work. It is just that simple and there is no way around the safety software to make them work.

                      There is equipment used in Hybrid solar systems that allow a pv array to charge a battery bank when the grid is down but those are very expensive and not part of a standard grid tie system.

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