Watt
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SI multiples of watt
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
Definition
The watt (symbol: W) is the SI unit of power defined as one joule of energy transferred or dissipated in one second (J/s). Its expression in terms of SI base units is m2·kg·s-3.
The watt is a common unit of electrical power, in which case one watt of power is expended when one ampere of direct current flows through a resistance of one ohm. 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).
For perspective, the approximate heat dissipation from an adult human is around 100 Wt. A typical kitchen microwave will be rated around 1000 W (1 kW) and a typical automobile's power output rating will be around 150 kW.
History
The unit is named for James Watt (1736-1819), the Scottish inventor, instrument maker and mechanical engineer, renowned for his improvements on the steam engine.
Conversions
1 watt = 1 joule/second
1 watt = 1 volt ampere
1 watt = 1·107 ergs/second
1 watt = 2.39006·10-4 kilocalorie (thermal)/second
1 watt = 9.48452·10-4 Btu (thermal)/second
Further Reading
- How Many? A Dictionary of Units of Measurement, Russ Rowlett, Center for Mathematics and Science Education, University of North Carolina at Chapel Hill
- SI base units and SI derived units, the Physics Laboratory, National Institute of Standards and Technology
- Energy conversion tool at unitconversion.org.