Off-Grid Solar Calculators

Inverters and charge controllers

Generator Sizing for Battery Charging

The generator has to cover the AC input of the charger, not just the DC watts into the battery. A 50 A 12 V charger is 600 W DC and closer to 850–1,000 W AC once efficiency and generator derate are included.

Inputs

A

System voltage of the battery or series string.

Results

DC charge power
600W
AC into the charger
667W
Suggested generator size
833W

Recommended gear

Engineering methodology

DC charge power = charge amps × battery volts (P = V × I). AC watts into the charger = DC watts ÷ charger efficiency. Suggested generator size = AC watts ÷ generator derate (0.80 is a common 1.25× headroom factor for altitude, heat and waveform).

Frequently asked questions

Why isn’t generator size just battery volts times charger amps?

That product is DC watts into the battery. The generator must supply AC watts = DC watts ÷ charger efficiency, then extra headroom. A 50 A 12 V charger is 600 W DC and often 800–1,000 W at the generator.

Do I need an inverter generator for a battery charger?

Many switch-mode chargers want a clean enough sine wave. Cheap modified-sine or heavily distorted sets can confuse chargers. Confirm with the charger maker. This formula only sizes watts.

What does the 0.80 generator derate mean?

It is 1 / 1.25 headroom: the set is not run at 100% of nameplate. Altitude, temperature and other loads on the same generator push the factor lower (for example 0.70).

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