Australian PC World compared the power consumption of two Linux distros and Windows Server 2008 on four 1U-servers:
Ensuring your servers stamp as small a carbon footprint as possible on the earth and in your data center can encompass everything from making sure they are shipped in recyclable packaging to hiring an analyst who can predict the total life-cycle environmental impact.
For this test, we examined power consumption as a way to judge whether Windows Server 2008 or Linux is, in fact, the 'greener' operating system. As the price of power hits record heights, power reduction mechanisms shipping within an operating system should play a key role in you energy conservation plan.
On average Linux used the least power, in some cases the power consumption was 12% lower than Windows Server 2008.