Battery capacity is quoted in amp-hours (Ah) but energy is measured in watt-hours (Wh). Enter the watt-hours and the battery voltage and this converts to amp-hours. Divide by voltage to go this way; multiply to go back.
This trips people up because a 100Ah battery does not hold a fixed amount of energy; it depends on the voltage. A 100Ah 12V battery stores 1,200 Wh, while a 100Ah 24V battery stores 2,400 Wh. When comparing batteries or sizing a solar setup, convert everything to watt-hours first so you are comparing energy, not amp-hours at different voltages. For home storage economics see home battery savings and solar battery storage.
Common questions
How do I convert Wh to Ah?
Divide the watt-hours by the battery voltage. For example, 600 Wh on a 12V battery is 600 divided by 12, which is 50 Ah.
Why does voltage matter for battery capacity?
Because amp-hours only tell you the charge, not the energy. The same Ah rating holds twice the energy at 24V as at 12V, so watt-hours is the fairer way to compare batteries.