It is the total energy comsumption over time that matters (kilowatt HOURS)so a few watts for a long time or a lot for a short one might end up as the same amount of CO2 at the powerstation, and the keepers of said station would far rather you had a steady demand anyway.
And those kW power supplies and kickarse cooling systems are just there for the once in a decade time that the whole shebang has to work its bits off for an hour or so? But we are talking music applications here, not the CPU popping up to crunch your VAT then going back to sleep. Music systems work at the top end of the computers power capability (why laptops screw ya!) or are you suggesting that musicians buy Deep Blue machines and use 1% of them so that they never kick in full welly?
I could well have all that last wrong, not seen the numbers yet but there is still Fredericks' point that it is very unlikely that the energy and resources used in making a new pc will ever be recouped by the (possible) energy savings.