AwesomeGeeks.com have a great tutorial on how to control the power usage of your Windows Home Server rig by using the add-in Lights Out.
“Lets face it, under most circumstances, you won’t need a home server on for 24 hours/day, 7 days/week. Whether you’ve built your WHS setup from scratch or you’ve re-appropriated a used computer for your file sharing needs, it’s essential to control the time that you can use the server for energy efficiency reasons. Though WHS doesn’t handle this natively, there is an excellent add-in that will allow you to control your access to WHS with just a few clicks.”
You can read their guide here.