jils (who has an excellent software development blog at dev.notoptimal.net) notes an efficient power supply can make a big difference. By the chiming in with this sort of thing is greatly appreciated. The blog format is not ideal it was just easy way to put stuff the web, but I really hope that people chime in ways of measuring and reducing energy usage, and for evaluating environmental decisions.
One other item of note on quantitating computer power usage, the power supply rating (say 350Watts) is the output number, which means you have to factor in the efficiency rating to determine how much power it will conceivably draw. While most computers don't draw the peak amount, you may be wanting to know this for peak loading if you are sizing a UPS or solar system.
So a 80% efficient 350W power supply can draw 350/.8 = 437.5 watts and a 60% efficient power supply can draw 583 watts. That one component can make a big difference especially if you leave your computer on all the time. Keep in mind that the .2 or .4 "inefficient" part of the power supply all goes to heat which means you have to run your a/c more.
Computer power supply efficiencies are actually variable based on load. So really in the end to know how much power you're drawing you have to measure it.
So like I always say, there's lots of things you can do to reduce your energy usage, but how do we evaluate the choices with our own constrained resources (read: wallet)
Is it better to spend an extra $100 to get a 80% efficient supply or is better to spend an extra $100 to get a lower power processor? The lower power CPUs out today are design to be in a 65 watt (thermal design envelope) meaning that they aren't meant to put out more than that. Generally that applies to a range of processors so the fastest ones will put out 65 watts (which is the same as the amount of power they draw) and the slower lower power ones might be in the high 50s. The regular power processors range from 80 as high as 120 watts (though those are typically workstation or extreme processors which aren't in the same purchase category). So lets say you save 30 watts by going with the low power cpu. For a 350 w power supply the difference between 70% efficient (about average) and 80% efficient is 125 watts.
Score one for the power supply.