How long will a 100Ah battery last?

A 100 Ah, 12 volt lead-acid battery runs a 100 watt load for about 6 hours. The battery holds 1,200 Wh, but lead acid is usually used only to 50% depth of discharge. That leaves 600 Wh of usable energy, and 600 Wh divided by 100 watts is 6 hours.

To find watt-hours, multiply amp-hours by volts. Then multiply by the share you plan to use, and divide by the load in watts. A bigger load drains it faster: double the watts and you halve the hours.

Lithium batteries can be cycled deeper, but deeper cycles still shorten their life. Battery University estimates about 300 cycles at 100% depth of discharge for an NMC cell, versus about 1,000 at 40%. The battery runtime calculator lets you set the depth of discharge and load.