Google has taken the unusual step of revealing how much electricity its huge server farms consume, as it seeks to counter concerns about the impact of booming internet use on global warming.
The search company said that it had used 2.26 megawatt hours of power in 2010. Had its demand for power been spread evenly over the year, that would have amounted to a constant supply of 258 megawatts, or about half the output of an average power plant.
Google also revealed its carbon footprint for the first time, putting its output of carbon dioxide last year at 1.46 million tonnes.
The disclosures mark one of the most concerted attempts yet by a big internet company to address questions about the impact of big data centres on global warming.
In a recent study, Verdantix, an energy and environmental research firm, criticized the internet industry for a "widespread lack of transparency", with only three of the 14 big global companies surveyed disclosing the extent of their greenhouse gas emissions.
Google had previously viewed details of its power use as competitively sensitive information, but had decided that greater disclosure could "drive additional transparency" by other big internet companies, said Rick Needham, Google's director of green business operations
The figures suggest that Google's data centres are more efficient than most used by big internet companies, said Jonathan Koomey, a consulting professor of civil and environmental engineering at Stanford University.
Mr Koomey estimated that Google operates some one million servers, or 3 per cent of the total number in use in data centres around the world, but that it consumes only around 1 per cent of electricity used by these centralized computing warehouses.
Google also said that 25 per cent of the electricity it used last year came from renewable sources, a proportion that is likely to grow to 30 per cent this year. That includes power bought under fixed 20-year agreements from two wind farms that is currently more expensive than electricity from the grid, but which the company expects to be a more economical source of energy later on, Mr Needham said.