## calculating amp draw of inverter

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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
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
• ... 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
• 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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