Battery Runtime Calculator

Find how long a battery will run a device. Enter the battery's amp-hours and voltage and the load in watts.

Battery type

Battery capacityAhBattery voltageV

LoadWWhat the device draws.Usable depth%Lead-acid 50%, lithium 80%.

Running AC devices through an inverter (about 90% efficient)

Battery runtime

6 hr

running 100 W

Stored energy (100 Ah x 12 V): 1,200 Wh
Usable at 50%: 600 Wh
Runtime at 100 W: 6 hours

Real runtime drops in the cold and as a battery ages. Lead-acid also gives less than its rating when drained fast.

How long will a battery last?

Multiply amp-hours by volts to get watt-hours, keep only the share you can safely use, and divide by the load. A 100 Ah, 12 V battery stores 1,200 Wh. Lead-acid is usually drained only to about 50%, which leaves 600 Wh, so a 100 W load runs 6 hours. A lithium battery used to 80% runs the same load for 9.6 hours.

Running AC devices through an inverter loses some energy; tick the inverter box to allow 10%. To refill the battery afterward, use the charging time calculator or the solar charging time calculator.

runtime (hours) = Ah x volts x usable share / load watts

Battery runtime chart

A 12 V battery, lead-acid at 50% usable.

12 V, 50% depth of discharge
Battery	50 W	100 W	300 W	500 W	1,000 W
50 Ah	6 hr	3 hr	1 hr	36 min	18 min
100 Ah	12 hr	6 hr	2 hr	1 hr 12 min	36 min
200 Ah	24 hr	12 hr	4 hr	2 hr 24 min	1 hr 12 min
300 Ah	36 hr	18 hr	6 hr	3 hr 36 min	1 hr 48 min

Frequently asked questions

How long will a 100Ah battery last?

At 12 V and 50% usable, a 100 Ah lead-acid battery gives 600 Wh: 6 hours at 100 W, 2 hours at 300 W. A lithium battery used to 80% gives 960 Wh, or 9.6 hours at 100 W.

Why can I only use half a lead-acid battery?

Draining lead-acid deeply and often shortens its life, so 50% is the common planning limit. Lithium batteries handle deeper discharges, so 80% or more is usual.

Does the battery last longer at low loads?

Somewhat. Lead-acid gives more of its rated capacity when drained slowly and less when drained fast, so high loads run shorter than the simple math suggests.