Watt-Hours Calculator

How it is calculated

The tool multiplies the capacity in amp-hours by the voltage in volts: Wh = Ah × V. The energy in kilowatt-hours is then kWh = Wh ÷ 1000. Watt-hours are shown with two decimals and kilowatt-hours with four. Use the battery's nominal voltage, the figure printed on its label, because the real voltage moves up and down as the battery charges and discharges. The tool does not check the numbers, so zero or negative values simply give zero or a negative result. A small battery rated in milliamp-hours must be converted first (mAh ÷ 1000 = Ah). The result is the energy that the label capacity represents, not the energy you can actually draw: conversion losses, discharge limits and battery ageing all reduce what is usable.

Common mistakes

• Typing the capacity in milliamp-hours. A power bank of 20,000 mAh is 20 Ah; entering 20000 gives a result 1000 times too large.
• Using the wrong voltage for the battery pack. Cells in series add their voltages, while cells in parallel add amp-hours, so enter the pack's total voltage with the pack's total Ah, not the figures of one cell.
• Comparing batteries by amp-hours alone. A 100 Ah battery at 12 V holds 1,200 Wh, but at 24 V it holds 2,400 Wh, so the energy differs even though the Ah rating is the same.
• Treating the result as usable energy. The label capacity is not fully available in practice, and an inverter or charger loses some energy as heat.
• Confusing watt-hours with watts. Watts measure power at an instant, and watt-hours measure energy over time, so a 1,280 Wh battery is not a 1,280 W battery.