Watt-hour
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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
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
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
| 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
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
External links
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