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.
Power draw is the consumption rate
Battery life is energy inventory divided by burn rate. A 74 Wh power bank feeding a 7.4 W router yields 10 hours; the same pack on a 37 W load lasts 2 hours. Calculating battery life from power draw makes that trade explicit before you buy a larger pack or shed loads. Watts must be real power at the battery terminals—not VA on a UPS sticker.
Work backward from a life target
Need 8 hours at 12 W? Required Wh = 12 × 8 = 96 Wh. Convert to mAh at your voltage: mAh = (Wh ÷ V) × 1,000. At 3.7 V that is ~25,950 mAh class—explaining why marketing mAh alone misleads when voltage differs. Size the pack from required Wh, then confirm life with the forward calculation.
Life vs. cycle life—different questions
This guide answers how long one charge lasts at a given watt draw—not how many years until chemistry fades. Deep daily cycles and heat reduce future Wh capacity; derate new-pack estimates when cells are aged. After life hours are calculated, use Battery Charging Time to see how quickly the pack refills at your charger watts.