I'm not sure that most of the energy is used during manufacturing. I have heard that a third of a computer's lifecycle energy use comes during manufacturing, that would seem more reasonable.
On the other hand, jaganath, you hit the nail on the head about making efficient use of the energy that a computer is using. I consider a computer using 120W that is hopefully finding a cure to alzheimers and cancer more efficient than a computer that is drawing 100W and doing absolutely nothing.
Plus, there is a LOT of idle time where my computer is doing nothing other than [email protected]
, probably half of the time that it is on, I'm not using it. I turn it off if I am out for a while or overnight, but even then, it is probably on 8 hours a day, and I am on it maybe 4 hours. It doesn't have effective ACPI for sleeping, so I just let it fold, and turn the monitor off myself.
Another point for the OP: If you don't mind buying a system, try a used G4 iBook or Mac Mini from Ebay. Not only will they be pretty cheap as they are not really wanted due to the switch to intel, but they are extremely power efficient, and buying used is more eco friendly.
I am on an iBook right now (my main machine is the one I talked about above with ACPI). You will be pretty limited in terms of heavy duty computing with a G4, but you could always use the 3800+ machine to do video encoding or the like.
Macs also have a sub 1W sleep mode that you can set to have come on after 10 minutes of idling. This sleep mode is almost as good as off, I don't have a watt meter, but I do know that the battery will survive the night in this sleep mode, with only 15% lost. I calculated it out, and it was like .4 watts or something like that. Figure 1-2 watts after the conversion losses on the power brick.
They are also really nice, quiet computers, at least the iBook G4. The iBook G4 has a 25mm fan, but it is normally in a mode that is so quiet, you won't hear it unless you are in a dead silent room, or you put your ear up to the machine. Once in a while if you are playing back h.264 and running [email protected]
at the same time, the fan will ramp up to loud mode, BUT it really doesn't matter if you are playing a lot of video, as you won't really hear it anyways.