Battery Runtime Calculator
Estimate how long a battery can power a load. Free, fast and easy to use.
How it works
Runtime is the usable energy stored in the battery divided by the power drawn. Usable capacity reflects how deeply you can discharge the battery (about 80–100 % for LiFePO4, around 50 % for lead-acid) and inverter efficiency covers conversion losses. Temperature and battery age reduce real runtime.
Formula
t [h] = Ah × V × usable % × efficiency % / W
Worked examples
- Battery capacity (Ah): 200 · Battery voltage (V): 12 · Load (W): 300 · Usable capacity (%): 90 · Inverter efficiency (%): 90 → Runtime: 6 h 28 min
- Battery capacity (Ah): 50 · Battery voltage (V): 12 · Load (W): 40 · Usable capacity (%): 50 · Inverter efficiency (%): 100 → Runtime: 7 h 30 min
Calculations run locally in your browser. Results are for information only — for engineering, electrical or financial decisions, verify the assumptions and applicable standards.
Frequently asked questions
How long will a 100 Ah battery last?
A 12 V 100 Ah battery stores 1,200 Wh. With 90 % usable capacity and a 90 % efficient inverter, it runs a 100 W load for about 9.7 hours.
What is the difference between Ah and Wh?
Ampere-hours (Ah) describe charge, watt-hours (Wh) describe energy. Multiply Ah by the battery voltage to get Wh: 100 Ah × 12 V = 1,200 Wh.