Amazing stats buried in a Wired magazine article:

"If it's necessary to waste memory and bandwidth to dominate the petascale era, gorging on energy is an inescapable cost of doing business. Ask.com operations VP Dayne Sampson estimates that the five leading search companies together have some 2 million servers, each shedding 300 watts of heat annually, a total of 600 megawatts. These are linked to hard drives that dissipate perhaps another gigawatt. Fifty percent again as much power is required to cool this searing heat, for a total of 2.4 gigawatts. With a third of the incoming power already lost to the grid's inefficiencies, and half of what's left lost to power supplies, transformers, and converters, the total of electricity consumed by major search engines in 2006 approaches 5 gigawatts.

The article continues . . .

That's an impressive quantity of electricity. Five gigawatts is almost enough to power the Las Vegas metropolitan area – with all its hotels, casinos, restaurants, and convention centers – on the hottest day of the year. So the annual operation of the world's petascale search machines constitutes a Vegas-sized power sump. In the next year or so, it could add a dog-day Atlantic City. Air-conditioning will be the prime cost and conundrum of the petascale era. As energy analysts Peter Huber and Mark Mills projected in 1999, the planetary machine is on track to be consuming half of all the world's output of electricity by the end of this decade."

For the complete article:
http://www.wired.com/wired/archive/14.10/cloudware_pr.html