Inverter Size Calculator
Enter the loads you'll run at once and your largest motor start — get the inverter size, the surge rating to look for, and the battery current it will pull.
Add up nameplate watts of the devices that run simultaneously. Not sure? Use the generator size tool to build the list, then bring the total here.
Starting draw of your biggest motor/compressor — typically 2–3× its running watts (fridge, pump, power tools).
85–92% is typical for quality inverters.
Recommended inverter size
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Choose a pure sine wave model for motors and electronics. Battery cables and fusing must match the DC amps shown — at 12 V they get thick fast.
How inverter sizing works
Two numbers matter on an inverter's spec sheet:
- Continuous rating — what it can supply all day. Size this at your simultaneous running watts + 20% headroom, rounded up to a standard size (300 / 500 / 1,000 / 1,500 / 2,000 / 3,000 / 5,000 W).
- Surge (peak) rating — what it can supply for a few seconds. Usually 2× continuous. It must cover your largest motor start on top of whatever else is already running.
The hidden third number is DC current: amps from the battery ≈ AC watts ÷ battery voltage ÷ efficiency. A 2,000 W inverter flat out on 12 V pulls nearly 190 A — that's why systems above ~1,500 W move to 24 V or 48 V.
Inverter FAQ
What size inverter do I need?
Simultaneous running watts + 20% headroom, rounded up to a standard size — then check the surge rating covers your largest motor start (2–3× its running watts).
Why do motors need a bigger inverter?
A fridge running at 150 W can pull 900–1,200 W for a second at startup. The inverter's surge rating must cover that spike.
How many amps will it draw from the battery?
DC amps ≈ AC watts ÷ battery voltage ÷ efficiency. 1,000 W on 12 V at 88% ≈ 95 A — large inverters belong on 24 V or 48 V systems.
Pure sine or modified sine wave?
Pure sine runs everything safely and is the default choice. Modified sine is cheaper but can overheat motors and damage some electronics.