In the category of “I wrote this a long time ago but forgot to post it”, Mindteck updated their 2009 white paper which explores power use of various PC platforms using Windows 7. In the new paper, they tested both sleep, idle, low use and high use scenarios and built a model to estimate cost savings by using a centralized power management policy.
Possibly one of the most interesting tests they did was the effect of processor chipset drivers on the power consumption of Windows XP and Windows 7. (table here is copy and pasted from the appendix of the doc, and painfully reformatted)
% Improvement Win7
P4 Updated Drivers
High-end Updated Drivers
What is notable is that while W7 beats XP on every test, the “out of box” differences are really significant. If you look closely, Windows 7 out of the box numbers are the same as with the updated drivers – this means that Windows 7 is taking care of the chipset drivers, even on older hardware. The same cannot be said for XP. And even with updated drivers (obtained manually), it’s still worse than Windows 7. How many folks have reinstalled Windows XP and not installed the chipset drivers. Quite a few I would guess.
The Mindteck green computing white paper can be obtained at http://www.mindteck.com/whitepapers/Enabling-Green-Computing.pdf
Well what about Windows 95 versus XP? I'm sure from 95, significant power consumption changes were made in XP. Anyway, I guess with every new version of Windows the power management get's better, as does everything else. Another reason for me to get a Windows 7 PC. =)