Battery Life Calculator
Estimate how long a battery will last from its capacity in mAh and the device's current draw, free and instant in your browser.
Enter Values
Minimum 0
Minimum 0
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the battery capacity in amp-hours and the load current in amps.
- Press Calculate for the runtime.
How it works
Runtime ≈ capacity (Ah) ÷ load (A). This is an ideal figure — usable runtime is lower because most batteries shouldn't be fully discharged.
Worked example
100 Ah at 5 A. 100 ÷ 5 = 20 hours (ideal).
Common mistakes
- Using full capacity for lead-acid — discharge only to about 50% to protect the battery.
- Forgetting inverter and wiring losses, which shorten real runtime.
Frequently asked questions
How is battery runtime calculated?
Runtime in hours = usable capacity in Wh ÷ load in watts. For amp-hours, multiply Ah by battery voltage to get Wh first.
How much of a battery can I actually use?
Lithium batteries tolerate roughly 80–100% depth of discharge, but lead-acid lasts far longer if you use only about 50%.
Why is real runtime shorter than the ideal?
Most batteries shouldn't be fully discharged (lead-acid often only to 50%), and high loads, cold and ageing reduce usable capacity. Treat the figure as a best case.
Related tools
- Solar Output Calculator
- Energy Cost Calculator
- Ohms Law Calculator
- Watt-hours to Amp-hours Calculator
- Battery Bank Size Calculator
- Watts to kWh Calculator
Explore more in Electrical, Electronics, Solar & Energy.
Cite this tool
Zerdly. (2026). Battery Life Calculator. Retrieved from https://www.zerdly.com/tools/battery-life-calculator/
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.