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AC and DC power consumption

  1. Mar 12, 2016 #1
    Hello, I just came across this site when looking for some info. Not sure if this is a stupid question but I would like to know what the correlation between AC and DC power is when comparing one device. For example if i had a a device connected to 120vac and it drew 500w then the amps would be 4.16. So if i use that same device and connect it to a power inverter on a 12vdc system, it would draw that same power through the inverter, but what would be the draw on the 12vdc side? Is it just simply 500w/12v=41.6amps. I am basically trying to figure how much AC power can be drawn from a 12vdc battery where 50a is the max current the can go through a dc meter. Thank you
  2. jcsd
  3. Mar 12, 2016 #2


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    12 V x 50 A = 600 W. An upper limit (some losses in the DC-AC converter)
    But my guess is the battery can't deliver that for very long ...
  4. Mar 12, 2016 #3


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    You are quite correct in your calculation of the power needed, but, as BvU indicates, the inverter is not 100% efficient. So you need to divide your calculated result by the efficiency.
    So if the inverter were 70% efficient, the current draw would be 41.6 / 0.70 = 59.4 A.
    But I don't know what a typical efficiency would be. (I have found a reference that suggests it can be in excess of 90%, which would mean 41.6/0.90=46.2A)

    Also, the inverter might not deliver the 120V AC you expect. If it delivers less, then your device might not draw its full 500W, then the 12V current could be less.
  5. Mar 12, 2016 #4
    Sorry, I was just going hypothetically. If everything was 100% efficient and a device plugged into a home outlet drew 500w over 120v, then the amps would be 4.16. In theory, if the same device ran 500w over 120v on a 12v battery. What are the numbers for the voltage, amps, and watts coming from the battery and going to the inverter? Is it simply calculating the amps because we know the voltage of the battery (12v), we know that the device used 500w over A/C, then would that give the answer or is it some random number? Hopefully that makes sense.
  6. Mar 13, 2016 #5


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    yes, as stated in your OP

    but remember, as you have been told, it's a long way from the real world, so don't try and use
    the figures to produce good info

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