According to Ou, "What I have found is that Windows 7 battery life is (very slightly) better than Windows Vista. However, it's not quite as good as Windows XP"
... Personally, I've found the battery life on my laptop dipped noticeably after installing Windows 7 RC on my Windows Vista laptop. What's the mileage for you early adopters out there, who have tried out Windows 7 already?
No one really knows what Windows 7 will be yet because it's not on the market, but XP's power management failures are well documented. It is amusing XP is held up as a gold standard and a shame that M$ designed ACPI specifically to harm free software but have been unable to deliver for their own customers. I get good power management with Debian on older hardware and Macs have always done better than PCs."