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 charger on a 12 V bank finishes bulk near 14.4 V, so it puts out about 720 W DC, draws about 800 W at the charger input, and needs roughly 840–1,210 W of generator depending on efficiency and derate.

Inputs

Units
A

Nominal bank voltage; charging power uses 1.2× this (14.4 V on 12 V).

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Engineering methodology

DC charge power = charge amps × charging volts (P = V × I). Charging volts are 1.2 × nominal (14.4 V, 28.8 V or 57.6 V): the end of bulk, where the charger still delivers full current at its highest voltage. 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, and at nominal volts it is too low: near the end of bulk a 12 V bank sits at about 14.4 V. The generator must supply AC watts = DC watts ÷ charger efficiency, then extra headroom. A 50 A charger on 12 V is about 720 W DC and about 1,000 W of generator at the default settings.

Do I need an inverter generator for a battery charger?

Many switch-mode chargers want a clean enough sine wave. Cheap open-frame sets with a distorted waveform or unsteady frequency 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). Compare the result with the generator’s rated running watts, not its peak or starting watts.

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