The power company sells energy. But "kilowatt" is not energy. It's a RATE of using energy.
Take a close look at your electric bill. The power company charges $0.11 per "kilowatt-hour". That's the energy you use in 1 hour if you're taking it at the rate of 1 kilowatt.
Remember that 1 watt means 1 Joule of energy per second.
So 1 kilowatt = 1,000 Joules per second, and
1 kilowatt-hour = (1,000 J/sec) x (3,600 sec/hr) = 3,600,000 J
= = = = =
Now for your radio . . .
(3 hours) x (3,600 seconds/hour) = 10,800 seconds
100 watts = 100 Joules/second
Energy used = (100 J/sec) x (10,800 sec) = 1,080,000 Joules
(1,080,000 J) x (1 kilowatt-hour/3,600,000 J) = 0.3 kilowatt-hour
(0.3 kilowatt-hour) x (11¢/kilowatt-hour) = 3.3¢ <==
= = = = =
Slightly different way:
(100 watts) x (3 hours) = 300 watt-hour
"kilo ..." means "1,000" of something, so
300 watt-hours = 0.3 kilowatt-hour
Cost = (11¢/kilowatt-hour) x (0.3 kilowatt-hour) = 3.3¢
