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How to calculate amp load based on watts for 12v system?

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  • How to calculate amp load based on watts for 12v system?

    My 230v inverter shows the load of the fridge plus fan at 140w. If I want to calculate how much amps this load is drawing from the battery do I do 140 divided by the pv voltage (eg 16v) or by the battery voltage (eg 13.1v) or just by 12v?

    I know the load is 140/230 = 0.6 amps but I want to figure out how much is being sucked out of the battery.

  • #2
    AC watts * 1.2 (80% efficient inverter) = DC watts so 140 *1.2 = 168dc watts / 13.1v = 12.8 amps dc or something pretty darn close

    I used your battery voltage, but generally, I would us the baseline bank voltage 12V and build a little extra bit of margin into the calculation.
    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

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    • #3
      Originally posted by Mike90250 View Post
      AC watts * 1.2 (80% efficient inverter) = DC watts so 140 *1.2 = 168dc watts / 13.1v = 12.8 amps dc or something pretty darn close

      I used your battery voltage, but generally, I would us the baseline bank voltage 12V and build a little extra bit of margin into the calculation.
      So which is more accurate to use battery voltage or system voltage with a margin of error (what's the margin exactly?)

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      • #4
        Nominal battery voltage is what you want to use, 12 volts. You want to use worse case. Electrical we measure with a micrometer, mark the spot with chalk, and cut with an axe.
        MSEE, PE

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        • #5
          Fire up the diesel generator to charge my flashlight batteries.
          2.2kw Suntech mono, Classic 200, NEW Trace SW4024

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