Watt-hour
From Wikipedia, the free encyclopedia
The watt-hour (symbol W·h) is a unit of energy. It is commonly used in the form of the kilowatt-hour, which is 1,000 watt-hours.
It is not an SI unit, despite being based on the watt. The SI unit of energy is the joule (J), equal to one watt-second. Even if the watt-hour were not redundant, it could not be an SI unit, as the hour is not. It is, however, a commonly used unit, especially for measuring electric energy.
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[edit] Definition
One watt-hour is the amount of electrical energy expended by a one-watt load drawing power for one hour. This is equivalent to 3,600 joules, the joule being the SI unit of energy. Thus a kilowatt-hour is 3,600,000 joules or 3.6 megajoules.
For example, a 60-watt light bulb uses 60 watt-hours of energy every hour. Similarly, a 100-watt light bulb uses 100 watt-hours in an hour. The kilowatt-hour is usually used by power companies to measure power usage, so a rate of, for example, US$0.10 per kilowatt-hour will cost US$0.10 to run a 1,000-watt light bulb for one hour. Similarly 1 kilowatt-hour could be defined as 1 "unit" of electricity.
[edit] Multiples
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 |
[edit] 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 |
[edit] Explanation
The watt-hour is derived from the multiplication of the SI unit of power (watt) and a 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. In addition, the typical consumer can readily conceptualize the notion of "using a kilowatt for one hour." 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.
Some sources mistakenly refer to watt-hours as "power." [1] A similar confusion can arise when describing daily energy use. For example, a solar cell array might have a peak power output of 100 watts, but in order to give an indication of its useable output as a function of time-varying conditions (such as the apparent daily solar motion, or dust collection on the surface), its typical output might be described as “1200 watt-hours per day.” Different writers may disagree as to whether this is a measure of power or energy usage.
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.
[edit] Footnotes
- ^ Spirit Status for sol 982-987. Mars Exploration Rover, Daily Updates. JPL. Retrieved on 2006-12-01. “Solar power is currently about 300 watt-hours.”
