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Off-Grid for Well Pump

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  • Off-Grid for Well Pump

    I have been a long time reader, but never had a question (hence why I never registered) until now. I have a degree in EE, so electrical concepts/math are second nature to me, but solar is something that just isn't taught in school . I am hoping you guys can help answer what will probably seem like basic knowledge to some of you veterans!

    My brother in law lives in upstate NY and wanted to supplement his propane generator with a small solar system for just his well pump. Even though he has a 500 gallon propane tank, there are times during the winter when rationing propane is appropriate when the power is out. Since water is essential to living, he wanted to focus on providing solar power for that first.

    He purchased the following:

    6: 100 watt, 12 volt solar panels (currently wired in parallel)
    1: 60 amp cMPPT charge controller
    4: AGM 12 volt, 100 AH batteries (currently wired in parallel)
    1: 3000 watt continuous/12,000 watt peak low-frequency split-phase inverter (12 volt input)
    His well pump is 1/2 HP, 220 volt. I don't have the make/model yet, but I have seen specs online where units of this type surge 35 amps on start (@220) and drop to 9 amp continuous.

    I already realize he should change the way the system is configured to take advantage of the MPPT charge controller, but I want to focus on figuring out why it is not working properly first.

    Now the bad news. When the pump starts, the inverter immediately goes into fault mode.
    I gave him my volt meter and had him check everything, so I know its wired correctly.
    I've checked all his wire gauges/lengths vs current draw/voltage drops and it all seems within spec.

    Let's do a little math and you guys tell me if I am correct.

    Surge watts for this pump would be 220 x 35 = 7,700
    Since the input voltage to the inverter is 12 volts, the inrush current would be 7,700 / 12 = 642 amps

    - Are those assumptions correct so far?
    - Regular car batteries have CCA (cold cranking amps) specs that would apply to this situation, but does this apply for AGM batteries for solar systems?
    - Is the inrush current exceeding what these batteries can supply and that is why it is failing? If so, can he swap the inverter to one that supports 48 volt input, wire the 4 batteries in series, which would drop the current from 642 amps to 160 amps?
    - If all he is using the inverter for is the 220vac well pump, does he really need split phase?

    Thanks in advance and I apologize if these are noob questions!!

  • #2
    642 amp draw...3 times the draw of most homes... but you didn't factor in the efficiency rating of the inverter...need to start over.

    If it were me, I'd figure out a use for what you have(not running a well pump) and buy a big cistern for the water that can gravity feed the house when the power is out
    1150W, Midnite Classic 200, Cotek PSW, 8 T-605s

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    • #3
      Sounds like the battery system is too small for even just starting that 1/2 hp pump.

      Also how long will your Brother in Law need to run that pump. That watt hour number is where the battery system should have been sized. Right now a 400Ah 12v battery system is only good for about 1000watt hours a day and that pump draws close to 2000 watt once it is started. So he can run it maybe 30 minutes at best once a day.

      While the 3000/12000watt inverter sound about right, IMO he got the wrong batteries and probably not enough panel wattage to recharge what he will be using just to run that pump.

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      • #4
        Originally posted by SunEagle View Post
        Sounds like the battery system is too small for even just starting that 1/2 hp pump.

        Also how long will your Brother in Law need to run that pump. That watt hour number is where the battery system should have been sized. Right now a 400Ah 12v battery system is only good for about 1000watt hours a day and that pump draws close to 2000 watt once it is started. So he can run it maybe 30 minutes at best once a day.

        While the 3000/12000watt inverter sound about right, IMO he got the wrong batteries and probably not enough panel wattage to recharge what he will be using just to run that pump.
        Running duration was something I asked him about also. He lives alone and does not use much water. The well pump feeds a pressure tank and cycles on water use. In a 24 hour day, I am not sure how many times the pump would cycle....probably not a lot. He can always add more holding tanks, thus reducing the amount of times the pump needs to cycle each day. He'll have to alter his lifestyle a bit during these periods of water conservation, but it can be done.

        The issue then moves back to surge current to get the pump running. This is where using 12 volts was a very bad choice in my opinion. It seems 48 volts would be the battery array voltage of choice to me, right .. in general?

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        • #5
          Originally posted by arasef View Post
          Running duration was something I asked him about also. He lives alone and does not use much water. The well pump feeds a pressure tank and cycles on water use. In a 24 hour day, I am not sure how many times the pump would cycle....probably not a lot. He can always add more holding tanks, thus reducing the amount of times the pump needs to cycle each day. He'll have to alter his lifestyle a bit during these periods of water conservation, but it can be done.

          The issue then moves back to surge current to get the pump running. This is where using 12 volts was a very bad choice in my opinion. It seems 48 volts would be the battery array voltage of choice to me, right .. in general?
          The higher voltage would have been better for a bigger battery system. I just don't think even a 48v 100Ah system (4 x 12v 100Ah wired in series) would have worked for that pump.

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