Watt
From The RadioReference Wiki
The watt is the unit of power. One watt is one joule of energy per second. It is named in honor of James Watt and is abbreviated W.
mW = milliwatt = 0.001 watts
W = watt = 1,000 mW
kW = kilowatt = 1,000 W
MW = megawatt = 1,000 kW = 1,000,000 W
GW = gigawatt = 1,000 mW = 1,000,000 kW = 1,000,000,000 W
See also WikiPedia:Watt.