Watt: Difference between revisions
Adding 100 Petawatts for total solar energy striking the earth's surface |
→Confusion of watts and watt-hours: watts per hour |
||
| Line 60: | Line 60: | ||
== Confusion of watts and watt-hours == |
== Confusion of watts and watt-hours == |
||
[[Power (physics)|Power]] and [[energy]] are frequently confused in the general media. Power is the rate at which energy is used (or generated). A watt is one joule of energy per second. For example, if a 100 watt light bulb is turned on for one hour, the energy used is 100 [[watt-hour]]s or 0.1 kilowatt-hour, or (60x60x100) 360,000 [[joules]]. This same quantity of energy would light a 40-watt bulb for 2.5 hours. A power station would be rated in watts, but its annual energy sales would be in watt-hours (or kilowatt-hours or megawatt-hours). A kilowatt-hour is the amount of energy equivalent to a steady power of 1 kilowatt running for 1 hour, or 3.6 [[megajoule]]s. |
[[Power (physics)|Power]] and [[energy]] are frequently confused in the general media. Power is the rate at which energy is used (or generated). A watt is one joule of energy per second. For example, if a 100 watt light bulb is turned on for one hour, the energy used is 100 [[watt-hour]]s ('''Wh''') or 0.1 kilowatt-hour, or (60x60x100) 360,000 [[joules]]. This same quantity of energy would light a 40-watt bulb for 2.5 hours. A power station would be rated in watts, but its annual energy sales would be in watt-hours (or kilowatt-hours or megawatt-hours). A kilowatt-hour is the amount of energy equivalent to a steady power of 1 kilowatt running for 1 hour, or 3.6 [[megajoule]]s. |
||
Terms such as 'watts per hour', which are sometimes used in the media, are meaningless in practice |
Terms such as 'watts per hour', which are sometimes used in the media, are meaningless in practice<ref>[http://www.windsun.com/Inverters/Inverter_selection.htm ''Inverter Selection'']. (n.d.) Northern Arizona Wind and Sun. Retrieved 27 March 2009.</ref>, unless referring to ''change'' of power per hour: Watts per hour ('''W/h''') is useful to charaterize the ramp-up speed of [[power plant]]s. E.g. a power plant that reaches a power of 1 MW from zero in 15 minutes has a ramp-up rate of 4 MW/h. [[Hydroelectricity|Hydroelectric]] power plants have a very high ramp-up speed, which makes them particularly useful in peak load situations. |
||
Often energy production or consumption is expressed as [[terawatt-hour]]s per year. As one year contains about 8,765.82 hours, one terawatt-hour per year equals about 114,080 [[megawatt]]s. |
Often energy production or consumption is expressed as [[terawatt-hour]]s per year. As one year contains about 8,765.82 hours, one terawatt-hour per year equals about 114,080 [[megawatt]]s. |
||
Revision as of 09:12, 20 April 2009
The watt (symbol: W) is the SI derived unit of power, equal to one joule of energy per second. It measures a rate of energy conversion.
A human climbing a flight of stairs is doing work at a rate of about 200 watts. A typical automobile engine produces mechanical energy at a rate of 25,000 watts (approximately 33.5 horsepower) while cruising. A typical household incandescent light bulb uses electrical energy at a rate of 25 to 100 watts, while compact fluorescent lights typically consume 5 to 30 watts.
One watt is the rate at which work is done when an object is moving at one meter per second against a force of one newton.
- .
By the definition of the units of ampere and volt, work is done at a rate of one watt when one ampere flows through a potential difference of one volt.[1]
Origin and adoption as an SI unit
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 General Conference on Weights and Measures in 1960 as the unit of power incorporated in the International System of Units (or "SI").
The watt is named after James Watt. As with every SI unit named for a person, its symbol starts with an upper case letter (W), but when written in full, it follows the rules for capitalisation of a common noun; i.e., watt becomes capitalised at the beginning of a sentence and in titles but is otherwise in lower case.
Derived and qualified units for power distribution
Nanowatt
The nanowatt is equal to one billionth of a watt. From a single star of magnitude +3.5 a square meter receives one nanowatt.
Microwatt
The microwatt (symbol:μW) is equal to one millionth (10-6) of a watt.
Milliwatt
The milliwatt (symbol:mW) is equal to one thousandth (10-3) of a watt. A typical laser pointer might output 5 milliwatts.
Kilowatt
The kilowatt (symbol: kW), equal to one thousand watts, is typically used to state the power output of engines and the power consumption of tools and machines. A kilowatt is approximately equivalent to 1.34 horsepower. An electric heater with one heating element might use 1 kilowatt. The average power consumption, over a year, of a household in the United States is about 8,900 kilowatt-hours[2].
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 supercolliders 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 U.S. nuclear power plants have net summer capacities between about 500 and 1300 MW.[3]
The earliest citing for "megawatt" in the Oxford English Dictionary is a reference in the 1900 Webster's International Dictionary of English Language. The OED also says "megawatt" appeared in a 28 November, 1947, article in Science (506:2).
Gigawatt
The gigawatt (symbol: GW) is equal to one billion (short scale) (109) watts. This unit is sometimes used with large power plants or power grids.
Terawatt
The terawatt (symbol: TW) is equal to one trillion (short scale) (1012) watts. The total power used by humans worldwide (about 15 TW) is commonly measured in these units. The most powerful lasers from the mid 1960s to the mid 1990s produced power in terawatts, but only for nanoseconds.
Petawatt
The petawatt (symbol: PW) is equal to one quadrillion (short scale) (1015) watts and can be produced by the current generation of lasers for time-scales of the order of femtoseconds (10-15). The total energy flow from the sun striking the surface of the earth is estimated at 100 petawatts.
Electrical and thermal
In the electric power industry, megawatt electrical (abbreviation: MWe[4] or MWe[5]) is a term that refers to electric power, while megawatt thermal or thermal megawatt [6](abbreviations: MWt, MWth, MWt, or MWth) refers to thermal power produced. Other SI prefixes are sometimes used, for example gigawatt electrical (GWe). [7]
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. The difference is due to the inefficiency of steam-turbine generators and the limitations of the theoretical Carnot Cycle.
Confusion of watts and watt-hours
Power and energy are frequently confused in the general media. Power is the rate at which energy is used (or generated). A watt is one joule of energy per second. For example, if a 100 watt light bulb is turned on for one hour, the energy used is 100 watt-hours (Wh) or 0.1 kilowatt-hour, or (60x60x100) 360,000 joules. This same quantity of energy would light a 40-watt bulb for 2.5 hours. A power station would be rated in watts, but its annual energy sales would be in watt-hours (or kilowatt-hours or megawatt-hours). A kilowatt-hour is the amount of energy equivalent to a steady power of 1 kilowatt running for 1 hour, or 3.6 megajoules.
Terms such as 'watts per hour', which are sometimes used in the media, are meaningless in practice[8], unless referring to change of power per hour: Watts per hour (W/h) is useful to charaterize the ramp-up speed of power plants. E.g. a power plant that reaches a power of 1 MW from zero in 15 minutes has a ramp-up rate of 4 MW/h. Hydroelectric power plants have a very high ramp-up speed, which makes them particularly useful in peak load situations.
Often energy production or consumption is expressed as terawatt-hours per year. As one year contains about 8,765.82 hours, one terawatt-hour per year equals about 114,080 megawatts.
See also
References
- ^ "Amps, Volts, Watts, Ohms". Retrieved 17 April 2007.
- ^ "The Physics Factbook". Retrieved 17 February 2009.
- ^ Nuclear Regulatory Commission. (2007). 2007–2008 Information Digest. Retrieved on 27 January 2008. Appendix A.
- ^ Cleveland, C. J. 2007. "Watt". Encyclopedia of Earth.
- ^ How Many? A Dictionary of Units of Measurement
- ^ http://apps1.eere.energy.gov/news/news_detail.cfm/news_id=12362
- ^ 'Megawatt electrical' and 'megawatt thermal' are not SI units,Taylor 1995, Guide for the Use of the International System of Units (SI), NIST Special Publication SP811 The International Bureau of Weights and Measures states that unit symbols should not use subscripts to provide additional information about the quantity being measured, and regards these symbols as incorrect. International Bureau of Weights and Measures. (2006). The International System of Units (SI). 132.
- ^ Inverter Selection. (n.d.) Northern Arizona Wind and Sun. Retrieved 27 March 2009.
External links
- Nelson, Robert A., "The International System of Units Its History and Use in Science and Industry". Via Satellite, February 2000.