[UPDATE 5:38PM - enabled full battery stretch2 mode and got more battery life]
It appears that Windows XP will beat Windows Vista on notebook battery consumption. Using the 6-cell battery rated for 56.16 watt*hours with an actual 58.26 watt*hours, the Lenovo X200 in its absolutely minimal1 power state will get barely over 8 hours of battery life while Windows Vista in its minimal power state gets about 6.5 7.6 hours. In these minimal states, it seems that Windows XP will allow the computer to drop down to 7.28 watts while Windows Vista only allows the same computer to drop to 7.67 watts.
With battery stretch turned off but Vista Aero still disabled, XP drops to 5.96 hours and Vista drops to 5.628 hours when I'm blogging on the computer with Wi-Fi and Bluetooth on. When I'm surfing the web and actively rendering web pages, XP drops to 5.5 hours while Vista drops to 5.22 hours.
It's very hard to make these measurements because there are so many factors that determine power consumption and battery life. Vista does seem to crank the CPU a little more but it's doing a lot more.
I've have always noticed that Windows Vista keeps the CPU slightly busier in idle than Windows XP and that may explain the power difference. Is this annoying? Sure, and it must give laptop designers fits because this makes it harder to hit the target battery life goals. Does this make me want to go back to XP? A little, but I do like the way the fonts look in Vista and I like the start menu better in the way you can launch programs by just typing in a few letters. A fresh install of Vista is stable enough these days that I've decided to stick with Vista, but the reduced battery life bothers me a little.
It would be interesting to see how a major Linux distribution will do on this laptop, but I don't have time for that right now with my work.
1. Minimal state meaning the display is on minimum brightness, Bluetooth and Wi-Fi radios off, hard drive spun down, CPU at minimum P and C state, and every other trick in the book. It's not that you can actually use the computer in this state, but it's a useful measurement to figure out one extreme of battery life.
2. Battery stretch turns off speakers, lowers color depth to 16 bit, lowers brightness to the minimum, lowers CPU to the minimum performance, and a few other minor things. Vista icons do look horrendous in this mode, but the X200 LCD is glare free so I can still watch movies on a dark air plane in this mode if I enable the audio.