Basically, there's been some sort of power regression in 2.6.35+ kernels. You can read a bit more about it at Phoronix. I'm not sure how the regression compares to Win7, but as long as you find a distro with a pre 2.6.35 kernel, you should be okay. Debian stable has 2.6.32 as it's base kernel.
But power consumption is more of an issue on laptops, which your server should not be. Is your power really expensive per kW-hr? Otherwise, you're probably only looking at a couple of bucks a month.
Additionally, make sure you turn off all unnecessary services.
Last edited by pljvaldez; 06-09-2011 at 06:01 PM.