Watt-hours to Kilowatt-hours (Wh to kWh)

1 watt-hour equals 0.001 kilowatt-hours exactly.

1 watt-hour = 0.001 kilowatt-hours

How to convert watt-hours to kilowatt-hours

To convert watt-hours to kilowatt-hours, multiply the number of watt-hours by 0.001. This factor is exact. To go the other way, multiply kilowatt-hours by 1,000.

kilowatt-hours = watt-hours × 0.001

Example: 10 watt-hours = 0.01 kilowatt-hours.

The 'calorie' on food labels is really a kilocalorie. The thermochemical calorie (4.184 J) is used here unless marked IT.

About these units

Watt-hour (Wh)

Metric (SI)Current unit

Exactly 3,600 joules, the energy used by a one-watt device in an hour. Battery capacities are quoted in watt-hours.

Pair a power unit with a unit of time and you get the watt-hour: one watt sustained for one hour, which works out to 3,600 joules. The watt itself was adopted at the second International Electrical Congress in 1889, in the same session that adopted the joule, so the two units share a birthday even though the watt-hour is the one people read off batteries.

Battery capacity is where it earns its keep. Multiplying amp-hours by the battery voltage gives an estimate of watt-hours, so a 2,500 mAh lithium-ion cell at 3.7 V holds roughly 9.25 Wh. That is why a 500 mA USB device runs about 3.7 hours on it rather than five: the cell voltage, not the 5 V from the port, sets the energy.

For scale, an alkaline AA battery holds about 9 kJ, which is 2.5 Wh. A 12 W LED lamp left on for an hour uses 12 Wh, and a 40 W appliance running for 25 hours uses 1,000 Wh.

1 watt-hour = 3,600 joules (exactly).

Sources: Wikipedia: Watt-hour; Wikipedia: Joule; Wikipedia: Orders of magnitude (energy)

Kilowatt-hour (kWh)

Metric (SI)Current unit

3.6 million joules, the energy used by a 1,000-watt appliance in an hour. The billing unit for household electricity.

Before it was a line on every power bill, the kilowatt-hour had a British nickname: the Board of Trade unit, abbreviated B.T.U. after the government body that regulated the electricity industry until 1942, when the Ministry of Power took over. In India it is still often called simply a unit, and a million of them, written MU, is a gigawatt-hour.

A kilowatt-hour is the energy of a 1 kW device running for an hour, which is 3.6 MJ, or about 3,412 British thermal units. A 100 W television watched for 10 hours uses one, and boiling a liter of water in an electric kettle takes about 0.1. In 2020 the average US household used 893 kWh a month, an average continuous draw of about 1.22 kW.

Utilities multiply the kilowatt-hours consumed by a price per kWh, and larger customers may also pay for peak demand. One common error is writing kW/h, which would mean kilowatts per hour, a rate of change of power, not energy.

1 kilowatt-hour = 3,600,000 joules (exactly).

Sources: Wikipedia: Kilowatt-hour; Wikipedia: British thermal unit; Wikipedia: Watt-hour

Related conversions

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