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  • Powering loads directly from the solar panels

    I am considering solar power. But I need to power critical loads from the solar panel directly when there is a power failure. A conventional transfer switch would isolate the inverter from the line.
    A small UPS could be used to convince the inverter that AC power is available. The inverter should then be able to supply power directly to the loads selected manually by the breakers in the power panel, just as if it were sending power back to the power company. It should not be necessary to have additional batteries, so long as I accept that the loads will be powered only when the sun is shining. A transfer switch and UPS can be obtained for about $500. Is there anything technically wrong with this approach?

  • #2
    Only thing wrong is it does not work. No problem.
    MSEE, PE

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    • #3
      Solar is a terrible power backup. Get a generator, a friend has one with auto test and auto transfer
      after startup when needed, natural gas powered. Bruce Roe

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      • #4
        Originally posted by Robert Dixon View Post
        I am considering solar power. But I need to power critical loads from the solar panel directly when there is a power failure.
        Then get an SMA inverter with the secure power feature. 2000 watts when the grid goes out (and the sun is out.) No need for kludges.
        A small UPS could be used to convince the inverter that AC power is available. The inverter should then be able to supply power directly to the loads selected manually by the breakers in the power panel, just as if it were sending power back to the power company.
        Won't work. If you are lucky, the grid tie inverter will just trip off-line when voltage exceeds limits. If you are unlucky, the resulting fire might be nasty.



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        • #5
          Originally posted by Robert Dixon View Post
          I am considering solar power. But I need to power critical loads from the solar panel directly when there is a power failure.
          Great, just get a bimodal system like those from OutBack power and you are all set.

          Originally posted by Robert Dixon View Post
          A conventional transfer switch would isolate the inverter from the line.
          Forget it, this is heading down the wrong path.


          Originally posted by Robert Dixon View Post
          A small UPS could be used to convince the inverter that AC power is available. The inverter should then be able to supply power directly to the loads selected manually by the breakers in the power panel, just as if it were sending power back to the power company. It should not be necessary to have additional batteries, so long as I accept that the loads will be powered only when the sun is shining. A transfer switch and UPS can be obtained for about $500. Is there anything technically wrong with this approach?
          OK just forget all this it will not work.

          If it did bad things would happen. there is very much technically wrong with it.
          OutBack FP1 w/ CS6P-250P http://bit.ly/1Sg5VNH

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          • #6
            Sorry to hijack your post but I was wondering how do I post a new topic in the general forum? I don't see any button to post a new topic. Thanks

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            • #7
              Originally posted by Robert Dixon View Post
              I am considering solar power. But I need to power critical loads from the solar panel directly when there is a power failure.
              Why?

              I live on the East Coast, so obviously we have a lot of power outages. Our grid power goes out at least once every month, sometimes it goes out 3 or 4 times per month. Most homes in our town have generators. Generators are much cheaper to install than solar power.

              On the downside of course is they consume fuel. One of our neighbors shares his fuel bills with me, he commonly spends $100 to 150 a month on fuel for his generator.

              We did not want to shift how much we pay for electric over to generator fuel, so we went with solar-power.

              I do not understand why you say that you need your critical loads to be fed directly from solar.
              4400w, Midnite Classic 150 charge-controller.

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              • #8
                The suggested SMA inverter seems to do what I am suggesting. No batteries. Thanks!
                I am familiar with generators as I had one in my previous house. My goal is to install a solar power system, but then why not take advantage of the existing power when the
                grid fails? It could power a freezer during the daytime, for example, and then the freezer would not thaw out at night. Batteries are very expensive. For those who said it will not work, please provide a detailed technical explanation of why not.

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                • #9
                  Originally posted by Robert Dixon View Post
                  ..... For those who said it will not work, please provide a detailed technical explanation of why not.
                  Solar power - fluctuates.
                  Birds fly overhead, shadow instantly cuts power, gear shuts down, then tries to restart against a high head, stalling the compressor.

                  No "inertia" or "Flywheel" effect, a generator can start a large pump motor because of the mass of the rotating gear. Solar has no inertia, no surge capacity. Maybe a fridge size bank of super caps can solve some of that, but at a huge expense.

                  Because of atmospheric conditions, 30% energy harvest can appear/disappear in seconds, so solar direct to loads, has to be massively over-sized, 8KW array, running 1Kw of outlet. Cloudy conditions will completely shut it down.

                  Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
                  || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
                  || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

                  solar: http://tinyurl.com/LMR-Solar
                  gen: http://tinyurl.com/LMR-Lister

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                  • #10
                    Originally posted by Robert Dixon View Post
                    The suggested SMA inverter seems to do what I am suggesting. No batteries. Thanks!
                    I am familiar with generators as I had one in my previous house. My goal is to install a solar power system, but then why not take advantage of the existing power when the
                    grid fails? It could power a freezer during the daytime, for example, and then the freezer would not thaw out at night. Batteries are very expensive. For those who said it will not work, please provide a detailed technical explanation of why not.
                    In my area the grid fails usually during storms so there is no sunlight to harvest. That is why a gen set or battery system works better as a backup power source then just hoping the sun comes out.

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                    • #11
                      So far I have been unable to find a dealer who will install a PV system with an SMA inverter in central Ohio. Any suggestions please? Thanks!

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                      • #12
                        Originally posted by Robert Dixon View Post
                        So far I have been unable to find a dealer who will install a PV system with an SMA inverter in central Ohio. Any suggestions please? Thanks!
                        Google is your friend. I also bet if you call SMA, they'd be willing to recommend a dealer who handles their products.

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                        • #13
                          If you have critical power needs then what happens when the power goes out at night? I personally wouldn't consider a freezer a critical need. A full chest freezer not opened will last at least 24 hours.

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                          • #14
                            The idea is that the solar panels would run a freezer for long enough to keep it cool during the day, and then it would coast thru the night. Potentially saving hundreds of dollars in frozen foods.
                            This is of course not the man reason for having the solar panels, but since the energy is available why not use it.

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                            • #15
                              Originally posted by Robert Dixon View Post
                              The idea is that the solar panels would run a freezer for long enough to keep it cool during the day, and then it would coast thru the night. Potentially saving hundreds of dollars in frozen foods. This is of course not the man reason for having the solar panels, but since the energy is available why not use it.
                              One low-ish cost option would be to use batteries, but use as little as two deep cycle batteries. The batteries would be your "flywheel" and let you coast through temporary solar interruptions. You would charge the batteries off a solar charge controller. You don't need a lot of battery since they would constantly be refreshed by your solar panels (minus brief interruptions). You could run a standard inverter off the batteries to directly run your freezer or if you want to power low power items in the house too, install an interlock plate on your breaker box (just like you would for a home generator setup). You would have to size your inverter and battery charge controller to your freezer average power use (keeping startup power in mind for the inverter).
                              A big bonus would be you could run LED/fluorescent lights and even keep a fridge going part time for a lot of the night.

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