I tried to find how much my Acer laptop takes, just as a comparison as I'm sure it uses a lot of power for a laptop, but its adapter only says "Output: 19 V [some squiggles] 9.5 A", so I don't know what the wattage of it is. As for my least power hungry Mac, that must be my Duo 230 - not only because I don't have an adapter for it at the moment, but because it's just so small and doesn't have a lot of the things that most laptops have.
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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)