The short answer is it depends. However, according to Propellerhead one way to get a fair idea of how much electricity your PC uses, and its contributing to global warming, is to consult the Journey Systems Power Supply Calculator.
It works by adding up the individual power consumption figures for all of the components in your PC, from the CPU to the CD or DVD drive. The site makes it very clear that the final figure — designed to help PC builders work out the sort of power supply they will need — is a theoretical maximum figure and unlikely ever to happen in normal use but it is a useful guide,
and if you add it to the power consumption of your monitor you should be able to work out roughly how much your PC is costing your to run. Okay, you know the drill by now, if you want to see hundreds more great tips like this one hurry over to PCTopTips