Mindteck released a power consumption data after testing "sleep, idle, low-use, and high-use scenarios" of various Windows PCs. "Also, they built a model to estimate cost savings (pic below) by using a centralized power management policy. Mindteck looked at the effect of processor chipset drivers on power consumption (in watts) of Windows XP and Windows 7 with varying driver configurations and older hardware."
Win7 lead in every single scenario. For Win7, consumption levels are actually the same as with updated drivers—this means that Win7 is taking care of chipset drivers, even on older hardware. The same cann't be said for XP, and even with updated drivers (obtained manually), it still performs worse than Win7."
More Info: Enabling Green Computing in the Enterprise -Mindteck (pdf) | Windows 7 Power Use Study