what I'm wondering is if it would be feasible to replace the power supplies in classic macs with more modern/efficient ones to reduce power usage. Obviously this would require some hardware hacking and may not be pretty in some cases, but it still may be worth investigating at least for classic Macs with larger size cases that modern power supplies could be retro-fitted into.

Click to expand...

So, to answer your question directly, what you propose would be possible, but the larger question is whether or not it's worth the effort. The calculations I offered show you roughly what the magnitude of savings could be. We're likely talking dollars saved per year, not hundreds, so if you're doing a purely economic analysis, the savings seem hardly worth it. Most of the power taken from the wall really does make it to the computer; only a small fraction is wasted by the power supply.

If you're thinking more along the lines of "think globally, act locally; every little bit helps," then the "ROI" analysis involves other considerations.