The current draw from a 12V or 24V battery when running an inverter depends on the actual load, not the inverter size. A quick rule is to divide watts by 10 for 12V systems or 20 for 24V systems.
For more accuracy, divide the load by the actual battery voltage and adjust for inverter efficiency (typically 85%).
This ensures you can correctly estimate battery drain and size your system safely.
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Calculating current draw from 12V and 24V systems when running an inverter
Documented in this article are common questions relating to the inverter draw (inverter amp draw or inverter current draw) for 12v (or 24v) batteries. If you’re looking for information relating to your 2000-watt inverter amp draw, we’ve got a breakdown of expectant voltage and efficiencies for a range of wattages below.
When using an inverter, the battery current draw depends on the load (watts), not the inverter rating.
Quick rule of thumb to calculate current draw:
12V systems: divide the load watts by 10
24V systems: divide the load watts by 20
Example: 300W load
12V system: 300 ÷ 10 = 30 Amps
24V system: 300 ÷ 20 = 15 Amps
Notes on wattage rating vs load:
- It is the actual load watts, not the inverter rating or (inverter size) that counts.
- A 1500 watt 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, which would be the same amount as a 1200 watt inverter at load capacity.
- A 2000w 12v pure sine wave inverter draws power based only on its load.
- To estimate battery life, think of load in terms of headlight watts:
- A 1000W inverter load - like running 10 x 100W driving lights which will drain a normal car battery very quickly
For a more accurate calculation of battery current:
Current (Amps) = Load Watts ÷ (Battery Voltage x Inverter Efficiency)
Inverter efficiency is typically 85% (0.85).
Example (12V system):
300W load at 12V → 300 12 ÷ 0.85 = 29.4A
300W load at 14V → 300 ÷ 14 ÷ 0.85 = 25.2A
Example (24V system):
300W load at 24V → 300 ÷ 24 ÷ 0.85 = 14.7A
300W load at 28V → 300 ÷ 28 ÷ 0.85 = 12.6A
This formula will give you more accurate results.