Inverter current draw

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

Documented in this article is a common question relating to the inverter current draw and a breakdown of expectant voltage and efficiencies. 

Question

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

Answer

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.

Note: Figures in brackets are for 24V systems.