Estimate how long a battery can run your load.

Amp-hours describe charge capacity. Multiplying amp-hours by battery voltage converts that rating to watt-hours, a more useful measure when a load is rated in watts.

A 12 V, 100 Ah battery contains a nominal 1,200 Wh. If you plan to use 50% of its capacity and an inverter is 90% efficient, about 540 Wh reaches the load. A steady 500 W load therefore has an idealized runtime of about 1 hour and 5 minutes.

Runtime estimates are planning numbers, not guarantees. Motors and compressors may also need a brief startup surge that is much higher than their running power.

Usable capacity is the share of the battery rating you intend to consume. Your battery manufacturer or system documentation should be the first source for that limit.

When a load is entered in watts, efficiency accounts for conversion between the battery and the device. When load is entered in amps, the calculator treats that as current drawn directly from the battery and does not apply a second efficiency loss.

Safety note: Confirm wire size, fusing, battery current limits, ventilation, and inverter surge capacity with manufacturer guidance or a qualified installer. Never short a battery.