Battery Life Calculator: Current or Power Load
Estimate battery runtime using mAh and mA at the same voltage, or convert nominal battery energy into runtime for a load in watts.
Your inputs
Choose the units and assumptions that match your data. Example inputs are illustrative.
Your result
Enter your values, then choose Calculate.
Results are calculated in your browser. Display rounding does not change the underlying calculation.
Calculation breakdown
Method, assumptions and worked example
How it works
Same-voltage current mode: hours = mAh ÷ mA × usable fraction. Power mode: nominal Wh = mAh × battery voltage ÷ 1,000; hours = Wh × usable fraction × conversion efficiency ÷ load watts. These are two distinct models and must not mix currents measured at different voltages.
Worked example
A 10,000 mAh battery supplying 500 mA at the same voltage with 80% usable capacity gives 16 hours. In power mode, 10,000 mAh at 3.7 V, 80% usable, 90% conversion efficiency and a 10 W load gives 2.664 hours.
Using the result
Rated mAh is tied to voltage and test conditions. A power bank’s cell-side mAh is not its output-side mAh. Aging, temperature, cutoff settings, peak loads and chemistry affect runtime. The usable percentage and efficiency are explicit assumptions; do not count the same loss twice.
Frequently asked questions
Why does voltage matter for a power bank?
Energy is capacity times voltage. Converter input and output currents can differ.
Does this model Peukert effects?
No. It uses a constant load and your entered usable fraction, not a chemistry-specific discharge model.
Method review and references
Formula and content reviewed on 10 October 2026. Worked examples are hypothetical. Methods and assumptions are explained on each page. Health estimates are not clinical measurements; symbolic tools are for entertainment.
The calculation or selection method is described above. Use the relevant school policy, supplier quote or measurement definition when choosing inputs.