Power draw in watts is the drain rate; stored watt-hours are the tank. This guide shows how to calculate battery life from power draw—capacity in mAh and voltage, load watts, and expected hours before empty.
Benefits
- Battery life (h) = Wh ÷ power_draw_W.
- Wh from mAh × V ÷ 1,000 ties label ratings to watt loads.
- Sweep power draw to see how life scales when loads change.
How it works
- Convert battery mAh and nominal voltage to watt-hours.
- Enter power draw in watts (measured or nameplate).
- Read battery life in hours—halve draw to double life, linearly.
FAQ
How do I calculate battery life from power draw?
Life (h) = Wh ÷ W. Find Wh = (mAh ÷ 1,000) × V. Example: 10,000 mAh at 3.7 V → 37 Wh. At 5 W draw → 37 ÷ 5 = 7.4 hours. At 15 W → 2.47 hours. Life is inversely proportional to watts.
Does higher power draw always shorten battery life proportionally?
In the Wh ÷ W model, yes—double watts halves hours. In practice, very high draw can trigger earlier cutoff from voltage sag and heat, so real life may be slightly shorter than the linear estimate at extreme currents.
How do I get power draw for an AC device on a battery?
Use the inverter input watts at the battery (or output watts ÷ inverter efficiency). A 40 W laptop on a 90% efficient inverter draws ~44 W from the pack. Enter that W value with your bank Wh or mAh × V.
Technical specifications
- Life_h = ((mAh ÷ 1,000) × V) ÷ power_draw_W.
- Inverse: required_Wh = target_hours × power_draw_W.
- Linear scaling: life × (W₁ ÷ W₂) when comparing draws.
- Related: battery-runtime-calculator, estimate-battery-duration-under-load.