Inverter current draw

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Note: Figures in brackets are for 24V systems.

How much current is drawn from the 12V (or 24V) battery when running an inverter?

The simple answer is- divide the load watts by 10 (20). E.g. For a load of 300 Watts,

the current drawn from the battery would be:

300 ÷ 10 = 30 Amps (300 ÷ 20 = 15 Amps)

NOTES:

• It is the actual load watts, not the inverter rating that counts. So a 1500W

inverter with a 500 Watt load would be 50 (25) Amps, not 150 (75) Amps.

The same inverter with a 1200 Watt load would draw 120 (60) Amps.

• For a quick idea of how long a battery will last without the alternator running,

think of the load watts in terms of headlight watts-

How long will my battery last with an inverter load of 1000 Watts?- About

as long as having 10 x 100W driving lights on.- You know a normal car

battery wont last long even with just 2 x 55W headlights and 4 x 5W

park/tail lights- 130W total.

• For a more accurate calculation of battery current: Divide load watts by actual

battery voltage, this will be in the range 12-14V (24-28V). Then to allow for

inverter efficiency, typically 85%, divide the figure by 0.85. Thus:

For a 300W load at 12V….300 ÷ 12 ÷ 0.85 = 29.4 Amps.

For a 300W load at 14V….300 ÷ 14 ÷ 0.85 = 25.2 Amps.

You can see the simple divide by 10 gives an easy “worst case” guide.

(Similarly:

For a 300W load at 24V….300 ÷ 24 ÷ 0.85 = 14.7 Amps.

For a 300W load at 28V….300 ÷ 28 ÷ 0.85 = 12.6 Amps.

You can see the simple divide by 20 gives an easy “worst case” guide.)

Updated: Nov 19, 2009 10:28