Battery Life Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/6/2026
Battery life is calculated with battery life (h) = capacity (Wh) ÷ power consumed (W). A 15 Wh phone battery with an average consumption of 1.5 W lasts about 10 hours of active use.
Explanation
This estimate rests on a simple dimensional-analysis relationship: a battery's capacity in watt-hours (Wh) represents the total energy it can deliver, and dividing it by the average power consumed in watts (W) directly gives a duration in hours. The practical difficulty isn't the formula itself, but estimating the actual average power consumed, which varies enormously with usage: a phone on standby consumes a few tens of milliwatts, the same phone playing video with the screen on can consume 1 to 2 W, and much more during intensive graphics-heavy gaming. If the battery capacity is known in milliamp-hours (mAh, the unit shown on most phones and external batteries) rather than watt-hours, simply convert it first using the formula Wh = (mAh × V) ÷ 1000, where V is the cell's nominal voltage (typically 3.7 V for lithium-ion) — the same underlying unit-conversion logic as our rule of three calculator. This same logic applies at every scale: from a power bank of a few watt-hours to an electric vehicle battery of several tens of kilowatt-hours, where it allows estimating driving range from the vehicle's average power consumption, a practical complement to our fuel trip cost calculator for combustion-engine vehicles.
Example: a phone's battery life while watching video
Inputs
Battery capacity: 15 Wh. Average consumption while watching video: 1.5 W.
Calculation
Battery life = 15 ÷ 1.5 = 10 hours.
Result
This phone can play video continuously for about 10 hours before the battery runs out.
Frequently asked questions
Why is real-world battery life often different from the estimate?
Power consumption is almost never perfectly constant: screen brightness, network signal strength, ambient temperature, and battery aging (which progressively loses capacity over charge cycles) all cause real consumption to vary. This calculator gives an estimate based on an average power, a good order of magnitude but not an exact prediction.
How do I convert a capacity from mAh to Wh?
Multiply the capacity in mAh by the cell's nominal voltage in volts, then divide by 1000. For example, a 3000 mAh battery at 3.7 V (standard lithium-ion) represents 3000 × 3.7 ÷ 1000 = 11.1 Wh. The nominal voltage is usually printed on the battery label or in the device's technical specifications.
Why is the result empty if the power consumed is zero?
A strictly zero consumption would mathematically correspond to infinite battery life, which has no practical meaning to display as a result. In reality, no electronic device in operation has a truly zero consumption: a power value must be entered, even a small one, matching its actual state (standby, active use, etc.).