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calculating amp draw of inverter

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  • jgeidl@sbcglobal.net
    If I have an inverter that providers 1000W @ 120V from a standard automotive 12VDC (13.8VDC) system. Can I calculate the amp draw by doing 1000/120= 8.3A to
    Message 1 of 3 , May 7, 2013
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      If I have an inverter that providers 1000W @ 120V from a standard automotive 12VDC (13.8VDC) system. Can I calculate the amp draw by doing 1000/120= 8.3A to see how much of a drain this device will place on the auto system?

      Thanks,

      Jim, K6JMG
    • Andy
      ... Sorry, no. The amperage ON THE 120 V SIDE will indeed be 8.3 A, if the load is resistive. But you wanted to know the current on the 12 V side. The
      Message 2 of 3 , May 7, 2013
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        > If I have an inverter that providers 1000W @ 120V from a standard automotive
        > 12VDC (13.8VDC) system. Can I calculate the amp draw by doing 1000/120= 8.3A
        > to see how much of a drain this device will place on the auto system?

        Sorry, no.

        The amperage ON THE 120 V SIDE will indeed be 8.3 A, if the load is
        resistive. But you wanted to know the current on the 12 V side. The
        inverter acts like a transformer. Voltage transforms one way; current
        transforms the other way.

        1000W out means something greater (in watts) into the inverter. For
        purposes of this exercise, let's say the inverter is 90% efficient at
        this power level, meaning it draws 1111 watts from the 12V side. The
        excess 111 watts goes into heating the inverter.

        1111 W / 13.8 V = 80.5 Amps, drawn from the car's electrical system.

        If the DC voltage drops, say to 12.0 V, then the current increases to
        92.6 A to deliver the same power.

        If the inverter's efficiency is less than 90%, the required input
        wattage, and amperage, must be greater.

        Regards,
        Andy
      • lists
        In article , ... And at those kind of current levels the internal resistance of the battery
        Message 3 of 3 , May 9, 2013
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          In article
          <CAN_saPLOC0uOd8ibqOTob7wKGG=L7-pUqxR_tqP1sn6Wg2irvg@...>,
          Andy <ai.egrps@...> wrote:
          > If the DC voltage drops, say to 12.0 V, then the current increases to
          > 92.6 A to deliver the same power.

          And at those kind of current levels the internal resistance of the battery
          will indeed cause the output voltage to drop - by quite a lot.

          --
          Stuart Winsor

          Midlands RISC OS and Raspberry pi show, 13th July 2013

          http://www.mug.riscos.org/show13/MUGshow.html
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