Inverter Size Calculator

How the size is worked out

An inverter has two ratings that matter. The continuous rating is what it can supply all day. The surge (peak) rating is what it can supply for a few seconds, enough to start a motor. List your loads and the calculator makes the same two checks the makers describe.

The running total is every load’s quantity times its running watts. The continuous rating needed is that total plus headroom, rounded up to the next watt. Headroom is a rule of thumb, not a code rule, so you can set it to 0, 25 or 50% or type your own.

The surge needed is the running total plus the single biggest starting figure. Loads rarely start in the same instant, so adding every start would oversize the inverter. If your loads do start together, tick the switch and the calculator adds all the starts instead.

Battery amps

The inverter pulls its power from the battery as direct current. The current is the AC watts divided by the battery volts and the inverter’s efficiency. The calculator shows it three ways: at your load, at the continuous rating and at the surge, each rounded up because the cable and the battery have to carry it. The Battery line on the plate shows the continuous-rating current at 12, 24 and 48 V side by side, and the table below the quick answer shows common sizes.

Doubling the battery voltage halves the current, which is why large inverters are built for 24 or 48 V. The efficiency box defaults to 85% because efficiency at full load is lower than the peak figure on a spec sheet; a higher number would understate the current.

Current rises as the battery voltage sags during discharge, so treat the figure as a minimum. Above 100 A the page tells you to use the inverter maker’s cable and fuse table. It gives no cable or fuse sizes of its own. The link beside it opens the voltage drop calculator with your battery volts and the amps filled in.

Checking a unit

Type a unit’s continuous and surge watts and the pilot lamp compares them with the need. It lights green when the unit covers both, and red when it misses either. Type watts, not VA: a rating in VA is higher than the real watts, and the maker’s datasheet gives both. Continuous ratings also fall in heat; some makers list a lower figure at 40 °C or above, so a unit that only just passes at room temperature may not hold up in a hot space.

Surge duration and wave shape

Many inverters hold their surge rating for about 5 seconds (Battle Born, and the Magnum MS-PAE spec). A motor that takes longer than that to start can trip the inverter even when the surge number looks big enough.

Choose pure sine for motors and sensitive electronics. Modified sine can be cheaper but is a poorer match for them.

What this estimate is

The result is an estimate from physics and published maker figures, with every step shown so you can check it by hand. It does not know your inverter’s low-voltage shutdown, your battery’s limits or your cable length. When the inverter’s manual disagrees with this page, the manual wins.

Worked example

1. Inputs: 1 × Motor load: 150 W running, 600 W starting; 1 × Heater: 1,000 W running, no starting burst; 1 × Laptop charger: 60 W running, no starting burst; Headroom: 25%; Battery: 12 V, inverter efficiency 85%; Starts: one at a time.
2. Running = 1 × 150 W + 1 × 1,000 W + 1 × 60 W = 1,210 W
3. Surge = 1,210 W + 600 W = 1,810 W (largest single start: Motor load)
4. Continuous need = 1,210 W × (1 + 25%) = 1,210 W × 1.25 = 1,512.5 W, rounded up to 1,513 W
5. DC amps at load = 1,210 W ÷ (12 V × 0.85) = 118.7 A
6. DC amps at rating = 1,512.5 W ÷ (12 V × 0.85) = 148.3 A
7. DC amps at surge = 1,810 W ÷ (12 V × 0.85) = 177.5 A

Questions people ask

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