PowerPedia:Watt
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The watt (symbol: W) is the SI derived unit of power, equal to one joule per second. A human climbing a flight of stairs is doing work at the rate of about 200 watts; a highly-trained athlete can work at up to approximately 2000 watts for brief periods. An automobile engine produces 25 000 watts (approximately 30 horsepower) while cruising. A typical household incandescent lightbulb uses 40 to 100 watts.
The watt-hour (symbol W·h) is a unit of energy. It is commonly used in the form of the kilowatt-hour (kW·h), 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. The watt-hour is not an SI unit, as the hour is not. It is, however, a commonly used unit, especially for measuring electric energy.
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Definition
One watt is one joule (the SI unit of energy) per second. A watts is equal to volts x amps. That is, if 1 volt of potential difference is applied to a resistive load, and a current of 1 ampere flows, then 1 watt of power is dissipated.
One watt-hour (abbreviated W*hr) is the amount of (usually electrical) energy expended by a one-watt load (e.g. light bulb) drawing power for one hour.
Humans tend to use watt-hours to measure energy rather than joules, because of intuitive reasons: For example, a light bulb draws power (units of watts) over a certain amount of time, resulting in a net amount of used energy: a watt has units of energy-per-time, and an hour is a unit of time, so when multiplied together they produce a unit of energy called the watt-hour.
Power companies produce energy - a good - which is often purchased by the customer in units of kilowatt-hours. Consider a setup with two 50W light bulbs (100W total) left on for 10 hours per day. The setup will consume 1kilowatt-hour per day. If a power company charges US$0.10 / kilowatt-hour, then those two light bulbs will cost US$0.70 over the course of a week. (See unit juggling for more information.)
1 watt-hour 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.)
Origin
The watt is named after James Watt for his contributions to the development of the steam engine, and was adopted by the Second Congress of the British Association for the Advancement of Science in 1889 and by the 11th Conférence Générale des Poids et Mesures in 1960.
SI multiples
| Multiple | Name | Symbol | Multiple | Name | Symbol | |
|---|---|---|---|---|---|---|
| 100 | watt | W | ||||
| 101 | decawatt | daW | 10–1 | deciwatt | dW | |
| 102 | hectowatt | hW | 10–2 | centiwatt | cW | |
| 103 | kilowatt | kW | 10–3 | milliwatt | mW | |
| 106 | megawatt | MW | 10–6 | microwatt | µW | |
| 109 | gigawatt | GW | 10–9 | nanowatt | nW | |
| 1012 | terawatt | TW | 10–12 | picowatt | pW | |
| 1015 | petawatt | PW | 10–15 | femtowatt | fW | |
| 1018 | exawatt | EW | 10–18 | attowatt | aW | |
| 1021 | zettawatt | ZW | 10–21 | zeptowatt | zW | |
| 1024 | yottawatt | YW | 10–24 | yoctowatt | yW |
Derived and qualified units for power distribution
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." 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.
Another derived unit that is sometimes used for household purposes is the kWh/yr, usually considered in annual energy consumption calculations, but with the dimensions of power, with 1 kWh/yr = 0.114 W. Note that this unit uses three units of time in one unit, namely second, hour and year, of which only the first is an SI unit.
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. as the energy goes the more kw to get per hour
Megawatt
The megawatt (symbol: MW) is equal to one million (106) watts.
Many things can sustain the transfer or consumption of energy on this scale; some of these events or entities include: lightning strikes, large electric motors, naval craft (such as aircraft carriers and submarines), engineering hardware, and some scientific research equipment (such as the supercollider and large lasers). A large residential or retail building may consume several megawatts in electric power and heating energy.
The productive capacity of electrical generators operated by utility companies is often measured in MW. Modern high-powered diesel-electric railroad locomotives typically have a peak power output of (3 to 5) MW, whereas a typical modern nuclear power plant produces a peak output on the order of 500 to 2000 MW.
According to the Oxford English Dictionary, the earliest citing for "megawatt" is a reference in the 1900 Webster's International Dictionary of English Language. The OED also says "megawatt" appeared in a 28 November, 1847, article in Science (506:2).
Watts electrical and thermal
Watt electrical (abbreviation: We) is a term that refers to power produced as electricity. SI prefixes can be used, for example megawatt electrical (MWe) and gigawatt electrical (GWe). Watt thermal (abbreviation: Wt or Wth) refers to thermal power produced; again SI prefixes can be used, for example megawatt thermal (MWt) and gigawatt thermal (GWt). These terms are used to disambiguate the electric output of a thermal power station versus the (larger) thermal output. For example, the Embalse nuclear power plant in Argentina uses a fission reactor to generate 2109 MWt of heat, which creates steam to drive a turbine, which generates 648 MWe of electricity.
 | This SI unit is named after James Watt. As for all SI units whose names are derived from the proper name of a person, the first letter of its symbol is uppercase (W). But when an SI unit is spelled out, it should always be written in lowercase (watt), unless it begins a sentence or is the name "degree Celsius". — Based on The International System of Units (http://www.bipm.org/en/si/si_brochure/chapter5/5-2.html), section 5.2. |
Related
- Orders of magnitude (energy)
- Conversion of units
- James Watt
- Orders of magnitude (power)
- Root mean square (RMS)
- Watt balance
- Power factor
- Declared net capacity (power plants)
References and external links
- Nelson, Robert A., "The International System of Units (http://www.aticourses.com/international_system_units.htm) Its History and Use in Science and Industry". Via Satellite, February 2000.
- http://ebtx.com/mech/ampvolt.htm
- Wikipedia contributors (http://en.wikipedia.org/wiki/Special:Recentchanges), Wikipedia: The Free Encyclopedia. Wikimedia Foundation. <http://en.wikipedia.org>.
- "Spirit Status for sol 982-987 (http://www.jpl.nasa.gov/missions/mer/daily.cfm?date=10&year=2006)".
