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Question on current draw from an inverter

  1. Feb 21, 2012 #1
    Hey everyone,

    I have a slight question regarding the current draw from an inverter

    If you had a 100W pump running on 230V obviously it's current draw is 100/230 which is 0.43 Amps

    but what if you were running the water pump from an inverter connected to a 12V battery. Would the current draw still be the same or would the inverter draw from the battery 100 Watts divided by 12V (resulting in 8.3 Amps being drawn - assuming 100% efficiency, i know thats not realistic but I'm simplifying) or would it still just draw 0.43 Amps and alter the voltage?


  2. jcsd
  3. Feb 21, 2012 #2


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    I would find the resistance......(v^2)/R=Power
    R=529 ohms


    12 volts=I*529

    New current = .023 amps.
    P=IV P=.23*12
    New power = 2.76 watts.

    The resistance of the load determines the current....not the watts. You can't assume the load will take 100 watts....you have to determine it's resitance first. See what I mean?

    If you take a 12 volt battery that does make the 8.33 amps....you will see that the resistance is now different in your load....and that is impossible. And yes, I'm not coverting the DC to inverter....but that isn't really relevant for the problem at hand.

    This motor would not even come close to starting.
  4. Feb 21, 2012 #3
    I think Tommy has an Inverter with 12V supply - but still produces 230VAC output for the pump - correct?

    In that case Tom - your second case is more accurate - it is a little more complicated - because the Motor will not have perfect power factor and the inverter will have losses as you mentioned. You need 100W to run the motor - 0.43A x 230V ~ 8.3A x 12V - Starting the pump requires higher current - so you would need an inverter of at least 200W - probably more.

    If the inverter is only taking 12VDC - and making ~ 8 VAC RMS - that would not run anything.
  5. Feb 21, 2012 #4


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    If that's the case...then I agree with Windadct. Keep in mind that the inverter would have to change the 12 volts to AC....then it would have to go thru a transformer to up the voltage to 230 volts. But I would guess that the fancy inverters have transformers built in.

    But still, the resistance of the load determines most of the factors.
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