The
watt (symbol:
W) is a
derived unit of
power in the
International System of Units
(SI). It measures rate of energy conversion. One watt is equivalent
to 1
joule (J) of
energy
per
second.
In terms of mechanical energy, one watt is the rate at which work
is done when an object is moved at a speed of one meter per second
against a force of one
newton.
- = = =
By the definitions of electric potential (
volt)
and current (
ampere), work is done at a rate
of one watt when one ampere flows through a potential difference of
one volt.
Examples of usage
A
human climbing a flight of stairs is doing
work at a rate of about . A typical
automobile engine produces mechanical energy at a
rate of (approximately 33.5
horsepower)
while cruising. A typical household
incandescent light bulb uses
electrical energy at a rate of 25 to 100 watts;
fluorescent lamps typically consume 5 to 30
watts to produce a similar amount of light.
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 (SI).
Multiples
- For additional examples of magnitude for multiples and
submultiples of the Watt, see Orders of magnitude
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 is equal to one millionth of a watt.
Milliwatt
The milliwatt is equal to one thousandth of a watt. A typical
laser pointer might output 5
milliwatts.
Kilowatt
The kilowatt 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 annual electrical energy
consumption of a household in the United States is about 8,900
kilowatt-hours, equivalent to an average power of about 1 kW.
Megawatt
The megawatt is equal to one million 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 commercial 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. On railways, modern high-powered
electric locomotives typically have a
peak power output of 5 or 6 MW although some produce much more -
the
Eurostar, for example,
produces more than 12 MW - while heavy
diesel-electric locomotives typically manage 3 to 5 MW, whereas
U.S.
nuclear power plants have
net summer capacities between about 500 and 1300 MW.
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 is equal to one billion watts. This unit is sometimes
used with large power plants or power grids. For example, as 2009,
the installed capacity of wind power in Germany is 25 GW.
The
Nuclear Plant
Doel has a capacity of 3 GW.
Terawatt
The terawatt is equal to one trillion watts. The total power used
by humans worldwide
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. The average stroke of
lightning peaks at 1 terawatt, but these strokes
only last for 30
microseconds.
Petawatt
The petawatt is equal to one quadrillion watts and can be produced
by the current generation of lasers for time-scales of the order of
femtoseconds (10
^{-15} s). Based on the average of
1.366 kW/m
^{2} of total solar irradiance the total
energy flow of
sunlight striking Earth's
atmosphere is estimated at 174 PW (cf.
Solar Constant). If all this power
were absorbed this would be equivalent to the Earth gaining mass at
a rate of 1.94 kg/s.
Electrical and thermal watts
In the electric power industry,
megawatt electrical
(abbreviation: MW
_{e} or MWe) is a term that refers to
electric power, while
megawatt
thermal or
thermal megawatt (abbreviations:
MW
_{t}, MW
_{th}, MWt, or MWth) refers to thermal
power produced. Other
SI prefixes are
sometimes used, for example
gigawatt electrical
(GW
_{e}).
For
example, the Embalse nuclear power plant in Argentina uses a fission reactor to generate 2109
MW_{t} of heat, which creates steam to drive a turbine,
which generates 648 MW_{e} of electricity. The
difference is due to the inefficiency of steam-turbine generators
and the limitations of the theoretical
Carnot Cycle.
Confusion of watts, watt-hours, and watts per hour
Power and
energy are frequently confused. Power is the rate at
which energy is generated and consumed. For example, if a light
bulb is turned on for one hour, the energy used is
100
watt-hours (W·h) or
0.1 kilowatt-hour, or 360 kJ. This same quantity of energy
would light a 40-watt bulb for 2.5 hours, or a 50-watt bulb
for 2 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
MJ.
Terms such as 'watts per hour' are often misused. Watts per hour
properly refers to the
change of power per hour. Watts per
hour (W/h) is useful to characterize the ramp-up speed of
power plants. For example, 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 and emergency situations.
Major energy production or consumption for a period is often
expressed as
terawatt-hours produced
or consumed during the period. The period used is often a calendar
year or a financial year. A terawatt-hour equates to a continuous
energy production or consumption of approximately 114 megawatts for
a period of one year.
See also
References
- 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
- Solar Energy Grew at a Record Pace in 2008
(March 25, 2009)excerpt from EERE Network News - U.S. Department of
Energy
- '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