Data centres across the world use about 30 billion watts of electricity, about the same as the output of 30 nuclear power plants, with digital warehouses in the US accounting for between quarter to a third of that load, The New York Times reported.

Those are huge numbers for an industry that often puts forth an image of environmental friendliness.

Consider Apple's soon-to-be-built research and development complex near its Cupertino, California, headquarters. The building, which will look something like a spaceship, will be entirely surrounded by a thick layer of trees and will be powered with its own energy centre that will run mostly off the grid.

Yet in a year-long investigation, The Times found that "most data centres, by design, consume vast amounts of energy in an incongruously wasteful manner."

It added: "Online companies typically run their facilities at maximum capacity around the clock, whatever the demand. As a result, data centres can waste 90% or more of the electricity they pull off the grid."

In addition, to prevent power failure, they rely on vast numbers of generators that spew diesel exhaust in amounts that often violate clean air regulations. In fact, many Silicon Valley data centres appear on California's Toxic Air Contaminant Inventory, a list of the biggest stationary polluters.

"It's staggering for most people, even people in the industry, to understand the numbers, the sheer size of these systems," Peter Gross, a man who has helped design hundreds of data centres, told The Times. "A single data centre can take more power than a medium-size town."

Not all are environmental abusers. For example, eBay's 245,000 square-foot data centre in Delta, Utah, is LEED Gold certified and is 50% cheaper to operate and 30% more efficient than previous eBay facilities, partly due to the 400,000-gallon water cooling cistern that collects rainwater and can keep the building cool for 7,000 hours without drawing any electrical current.