Enter the battery’s usable energy capacity in watt-hours, its starting and target charge percentages, and the charger’s average input watts. An explicit overall efficiency connects input energy to energy stored in the battery.

Formula: battery Wh added = usable Wh × (target % − start %) ÷ 100; input Wh = added Wh ÷ (efficiency % ÷ 100); hours = input Wh ÷ average input W

Worked example

For a 1,000 Wh usable battery going from 20% to 80%, 600 Wh must be stored. At 80% overall efficiency, the input supplies 750 Wh; an average 250 W input would take 3 hours.

Power is an average input assumption

OpenStax relates energy, power and time through energy = power × time. The entered watts are average electrical power into the charger during this period, not a voltage or a maximum label rating. If power changes, use a measured or defensible average.

Efficiency is not free energy

The efficiency input represents all losses between the measured input and stored usable battery energy. Use a value relevant to the device and state-of-charge range; this page does not infer one from battery chemistry or charger type.

Charging often tapers

Real charge controllers may reduce power near the target level, and temperature or cell balancing can add time. This is an energy-budget estimate under constant average input power, not a guaranteed completion time or a charging procedure.

Explore the related calculation

For a different starting point, try the battery runtime calculator or return to the main calculator. Browse the complete tool directory for this site.

Reference and corrections

The definitions and assumptions above follow these sources; the examples and calculator arithmetic are original to this page.

Found a problem? Email contact@wattstoamps.app.

Battery charge time calculator

Enter your values

Result