A Harvard University physicist has claimed that every Google search uses half the energy it takes to boil a kettle – around seven grams of CO2.

The claims are made in research that is not yet peer-reviewed but has been submitted for publication to the US Institute of Electrical and Electronics Engineers.

Alex Wissner-Gross told The Sunday Times that Google operates a number of datacentres which consume a huge amount of power, and that servers all over the world are often used for a single search.

"Google is very efficient but its primary concern is to make searches fast and that means they have a lot of extra capacity that burns energy," he said.

Google does not make public figures on its energy use or the amount of servers it has around the world.

But it strenuously denies the seven grams figure, saying that, although a search may use computing power from a number of different servers around the world, it only uses their time for a fraction of a second.

"In terms of greenhouse gases, one Google search is equivalent to about 0.2 grams of CO2," Urs Hölzle, senior vice president of operations wrote on the official Google blog yesterday.

"The average car driven for one kilometre produces as many greenhouse gases as 1,000 Google searches."

Analyst firm Gartner estimates that datacentres account for 0.5 per cent of global CO2 emissions, a figure that it expects to rise dramatically as IT use around the world increases.