Excerpt from michaelbluejay.com - Mr. Electricity

A typical desktop computer uses about 65 to 250 watts. To find the figure for your particular computer you can contact the manufacturer (not me), or see my section on measuring electrical use.

With most devices you can look at the label to see how much energy they use, but that doesn't work so well with computers because the label gives the theoretical maximum, not the typical amount used. A computer whose label or power supply says 300 watts might only use about 70 watts when it's actually running, and only 100 even in peak times with serious number-crunching and all the drives spinning.

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