Battery Watt-Hours Calculator

How to use the battery watt-hours calculator

Enter the amp-hour capacity and the battery voltage, and you get the energy in watt-hours (and kilowatt-hours). Watt-hours = amp-hours × volts. This single number is the only fair way to compare batteries, because amp-hours on their own are meaningless without the voltage they sit at.

Here’s why amp-hours alone mislead: a “100 Ah” label means nothing until you know the voltage. A 100 Ah 12 V battery holds 1,200 Wh, while a 100 Ah 48 V battery holds 4,800 Wh — four times the energy behind the identical amp-hour figure. Comparing two batteries by amp-hours across different voltages is comparing nothing at all.

Watt-hours fixes that. Convert every battery to watt-hours (amp-hours × volts) and you can compare them directly, whatever the chemistry or voltage. It’s also the unit you need to match a battery to an energy load: a device that uses 1 kWh a day needs about 1,000 Wh of usable battery — before you derate for depth of discharge.

Watt-hours is also exactly how airlines regulate lithium batteries in carry-on bags. Under 100 Wh flies freely; 100–160 Wh needs airline approval (usually a two-spare limit); over 160 Wh is barred from passenger flights altogether. A 100 Ah 12 V battery is 1,200 Wh — nowhere near carry-on legal — while a typical 10,000 mAh power bank (about 37 Wh at 3.7 V) is fine. The carry-on size checker covers the dimensional side of packing.

Pair this with the battery-life calculator, which works in amp-hours and load, to go from energy to runtime — and remember to derate for usable depth of discharge, since the full watt-hours are rarely all available. Watt-hours tells you how much energy is in the pack; depth of discharge tells you how much of it you should actually use.

The formula

Energy is capacity times voltage. Divide by 1,000 for kilowatt-hours:

watt-hours = amp-hours × volts
kWh = watt-hours ÷ 1000

Worked example with the defaults — 100 Ah × 12 V = 1,200 Wh, or 1.2 kWh. A 50 Ah 24 V battery holds the same 1,200 Wh; a 200 Ah 48 V battery holds 9,600 Wh.

This is why amp-hours alone can’t compare batteries: a “100 Ah” 48 V pack stores four times the energy of a “100 Ah” 12 V one. Always compare in watt-hours.

Frequently asked questions