Hyper-V makes it possible to consolidate servers onto a much smaller number of physical machines, significantly reducing power consumption without unduly sacrificing performance. Multiple virtual machines can run on a single physical machine without consuming significantly more power than a standalone server while keeping comparable throughput. This means you can add virtual machines at essentially no power cost, as dictated by your hardware and performance needs. The savings continue to scale with the number of servers you are able to virtualize. Running 4 virtual machines means saving the equivalent power output of three physical servers; running 10 virtual machines means saving the equivalent power output of 9 physical servers.
The Windows Server 2008 Power Savings White Paper details some of the savings:
Read more about Energy Efficiency Best Practices in Microsoft Data Center Operations on the Microsoft Environment site.
Virtualization ROI Tool
Five Ways to Reduce Data Center Sever Power Consumption
"In the Data Center, Power and Cooling Costs More Than the IT Equipment It Supports." Electronics Cooling.
The Green Grid Data Center Power Efficiency Metrics: PUE and DCiE.