It's not just the computers themselves that use all this power: the combined heat output of all these servers, hard drives and network gear is so large that massive air conditioning is required to keep it all from overheating. "Cooling is about 60 percent of the power costs in a data center because of inefficiency," said Hewlett Packard executive Paul Perez in Data Center News. "The way data centers are cooled today is like cutting butter with a chain saw." Cooling capacity is often the limiting factor of how big these systems can be -- I've talked with more than one engineer whose data center facility sat half empty or more; even though there was plenty of room for more servers, the building's air conditioning was maxed out.Link (Thanks, Alex!)
Update: John sez, "There's an EPA report which estimates that 'servers and data centres' accounted for 1.5% of US energy consumption in 2006. But that's a long way from 'data centres = 1.5% of the whole world's energy consumption.'"
I write books. My latest is a YA science fiction novel called Homeland (it's the sequel to Little Brother). More books: Rapture of the Nerds (a novel, with Charlie Stross); With a Little Help (short stories); and The Great Big Beautiful Tomorrow (novella and nonfic). I speak all over the place and I tweet and tumble, too.