I don't rally have a clue how much power any of my computers use, nor how to find out. I don't pay the electricity bill at home anyway! 
I don't leave computers running all the time though as that just causes unnecessary wear on them, and also my Acer in particular is very noisy and heats up quite a lot if you leave it running - I put them in standby mode or shut them down if I don't need to use them for a while. Presumably my G3 uses the most power of any of my Macs, given that it's the only tower and has lots of drives in it.
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Main system: custom C2D 2.66GHz tower/4GB RAM/2600XT 512 MB/500GB HD/XP Pro x86+Vista Business x64
Main Mac: Beige G3 minitower (Sonnet G4 500MHz/640 MB DVD/USB/Radeon 32 MB/OS 9.2.2+10.4.11)
I don't leave computers running all the time though as that just causes unnecessary wear on them, and also my Acer in particular is very noisy and heats up quite a lot if you leave it running - I put them in standby mode or shut them down if I don't need to use them for a while. Presumably my G3 uses the most power of any of my Macs, given that it's the only tower and has lots of drives in it._________________
Main system: custom C2D 2.66GHz tower/4GB RAM/2600XT 512 MB/500GB HD/XP Pro x86+Vista Business x64
Main Mac: Beige G3 minitower (Sonnet G4 500MHz/640 MB DVD/USB/Radeon 32 MB/OS 9.2.2+10.4.11)