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An average desktop computer uses between 60 and 300 watts . It is very difficult to know exactly how much computers use on average because there are so many different hardware configurations. Also it highly depends on the usage of your computer. The computer power supply is not an accurate way to measure energy use because the power supply output is advertised as the max amount of watts a power supply can output. The electricity usage of a computer also heavily depends on the video card, a high end video card can use a lot of power and having more than one (SLI or Crossfire mode) during heavy gaming or 3d rendering can use a large amount of energy. It is estimate that an average modern desktop PC will use approximately 100 watts of power, not including the display screen.