Inverter Size Calculator
Sizes an inverter from your running load and the worst starting surge, and shows the DC current it will pull from the battery — the number that sets your cable and fuse sizes.
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The formula
continuous = running watts × (1 + headroom) surge rating ≥ largest motor starting watts DC amps = running watts ÷ efficiency ÷ system voltage
Two ratings matter and they are checked separately. The continuous rating must carry the steady load; the surge rating must survive the moment a compressor starts. An inverter that passes one and fails the other will trip in normal use.
What to watch out for
Inverters have two ratings and both have to pass:
- Continuous — what it can supply indefinitely. Size it above your realistic simultaneous load, not the sum of everything you own.
- Surge — usually 2× continuous for a few seconds. Motors draw 3–7× their running watts at startup, so a 700 W fridge compressor can demand 2,500 W briefly.
The DC current figure matters more than people expect. A 3,000 W inverter at 12 V pulls over 270 A — that is 70 mm² cable and a 300 A fuse. The same inverter at 48 V draws 68 A. This is usually what decides system voltage.
Frequently Asked Questions
What size inverter do I need?
Add up what genuinely runs at once, add 20% headroom, and check the surge rating covers your largest motor start. For 1,500 W of running load that is a 2,000 W inverter with at least 3,000 W surge.
What is inverter surge rating?
The power it can deliver briefly — typically twice continuous for a few seconds. It exists because motors draw several times their running current at startup.
How much DC current does an inverter draw?
Running watts ÷ efficiency ÷ battery voltage. A 2,000 W load at 92% on 48 V is about 45 A; the same load at 12 V is 181 A, needing far heavier cable.
Should I oversize the inverter?
A little. Inverters are least efficient at very low load, so a hugely oversized unit wastes standby power all day. 20–25% headroom is the usual balance.