A complete desktop computer setup, including speakers, a printer and an Internet modem, uses an average of 200 watts per hour; of this, the speakers use 20 watts an hour, the modem 10 watts and the printer 5 watts. Laptops, by comparison, use between 50 and 100 watts an hour.
Running a computer for 8 hours a day uses about 50 kilowatts a month, which is 600 kilowatts a year. Laptops use one-quarter to one-third of this amount of electricity. Placing a computer in stand-by or sleep mode drops electricity use to about one-third of active use. Leaving a computer running 24 hours a day would use 144 kilowatts a month, or 1,728 kilowatts per year.