About battery runtime calculator
How long a battery lasts depends on its capacity and how much power the load draws. Enter the battery's Ah and voltage, the usable charge and the efficiency.
Formula
Runtime (hours) = battery Wh × usable share × efficiency ÷ load in watts. A 12 V 100 Ah battery holds 1,200 Wh; with 50% usable and 85% efficiency it delivers 510 Wh, so a 100 W load runs for about 5.1 hours.
Usable charge
Lead-acid inverter batteries should not be drained below about 50 percent, while lithium batteries can usually use 80 percent or more.
Another worked example
A small 12 V, 7 Ah battery stores 84 Wh. With 80 percent usable and 85 percent efficiency, it supplies about 57 Wh. A 20 W router and modem would run for about 2.9 hours on it.
Extending runtime
- Reduce the load, since runtime is inversely proportional to power.
- Use a larger battery or connect identical batteries in parallel.
- Keep batteries cool, because heat shortens their life.
