Network World ran a series of independent tests and discovered that Red Hat Enterprise Linux uses 12 percent less power than Windows Server 2008. The greener operating system by a significant margin? Linux.
As with any benchmarking test, "your mileage may vary" but it's consistent with other findings that Linux is the greener operating system.
So, if you want to save money on rising energy costs, your best bet may well be Linux. Freedom and cost savings...all in one little penguin.