How much electricity do computers use?
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• A typical desktop computer uses about 65 to 250 watts.
• Add another 20-40 watts for an LCD monitor, or about 80 watts if you have an old-school 17" CRT.
• Most laptop computers use about 15-60 watts, far less than desktops.
• As long as your computer goes into sleep/standby when you're not using it, your computer doesn't use squat for electricity, compared to the rest of your household.
Source: http://michaelbluejay.com/electricity/computers.html