Its interested to note that there are a lot of computer companies making a concerted effort to reduce energy consumption, but how much energy do computers really use?
I can really only comment on personal and small business use of computers, and its not too too much. Its enough to encourage me turn off machines to save a few dollars a month, but compared to heat and air conditioning, its not too much, as far as I can tell. In addition to that, there is a significant expense in the transmission of electricity to my home and office, so even if a computer doesn't use that much power, there is also power consumed in getting the power there.
Since getting power to computers requires power, its another reason why I like the idea of solar powered computers. But the problem with that is that you'd of course need a power supply for the night time.