A good article from Network World on using virtualization to boost efficiency and save on energy costs:

The U.S. Environmental Protection Agency recently proclaimed that data centers consumed 61 billion kilowatt-hours of electricity in 2006. That’s roughly 1.6% of total U.S. electricity consumption and is worth about about $4.5 billion.

and

Applying virtualization technologies across all system assets, including servers, storage and network devices, can allow companies to tap into unused capacity without adding resources that draw more power. The most energy-efficient equipment is equipment that’s no longer in use, whether it’s a server, a router or a storage device.

The full article is here.