The watt-hour (symbol W·h) is a unit of energy. The more common kilowatt-hour is 1,000 watt-hours.

Neither is an SI unit because they contain the non-SI unit hour. The SI unit of energy is the joule (J), equal to one watt-second.

Definition

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One watt-hour is equivalent to one watt of power used for one hour. This is equivalent to 3,600 joules. Thus a kilowatt-hour is 3,600,000 joules or 3.6 megajoules.

For example, a sixty watt light bulb uses 60 watt-hours of energy every hour. Similarly, a 100 watt light bulb uses 100 watt-hours in an hour (and 50 in thirty minutes).

Multiples

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Milli-, kilo-, mega-, giga-, and tera- are the most-used prefixes.

Multiple	Name	Symbol		Multiple	Name	Symbol
100	watt-hour	W·h
103	kilowatt-hour	kW·h		10–3	milliwatt-hour	mW·h
106	megawatt-hour	MW·h		10–6	microwatt-hour	µW·h
109	gigawatt-hour	GW·h		10–9	nanowatt-hour	nW·h
1012	terawatt-hour	TW·h		10–12	picowatt-hour	pW·h
1015	petawatt-hour	PW·h		10–15	femtowatt-hour	fW·h
1018	exawatt-hour	EW·h		10–18	attowatt-hour	aW·h
1021	zettawatt-hour	ZW·h		10–21	zeptowatt-hour	zW·h
1024	yottawatt-hour	YW·h		10–24	yoctowatt-hour	yW·h

Conversions

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from / to	Joule	Watt-hour	Electronvolt	Calorie
1 J = 1 kg m2 s-2 =	1	0.278 · 10−3	6.241 · 1018	0.239
1 kW h =	3.6 · 106	1000	22.5 · 1024	0.860 · 106
1 eV =	0.1602 · 10−18	44.5 · 10−27	1	33.8 · 10−21
1 cal =	4.1868	1.163 · 10−3	0.261 · 1018	1

Explanation

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The watt-hour is derived from the multiplication of the SI unit of power (watt) and the non-SI unit of time (hour).

The kilowatt-hour is commonly used for electrical and natural gas energy. Many electric utility companies use the kilowatt-hour for billing. This is a convenient unit because the energy usage of a typical home in one month is several hundred kilowatt-hours. Megawatt-hours are used for metering of larger amounts of electrical energy. For example, a power plant's daily output is likely to be measured in megawatt-hours.

The Board of Trade Unit or B.O.T.U. is an obsolete UK synonym for kilowatt-hour. The term derives from the name of the Board of Trade that regulated the electricity industry. The B.O.T.U. should not be confused with the British thermal unit or BTU, which is a much smaller quantity of thermal energy.

See also

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• Orders of magnitude (energy)

External links

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• Conversion of W·h in other units

Units of energy | Electric power

Киловатчас | Wattstunde | Kilovatio-hora | Kilowatt-heure | 와트시 | Wattora | Kilowattuur | キロワット時 | Kilowattime | Kilowattime | Kilowatogodzina | Quilowatt-hora | Киловатт-час | Kilovatna ura | Kilowattitunti | Wattimme | 千瓦·時