It is just one example of the growing concern over the increasing power consumption and environmental impact of computers. A study found that the power consumption of data centers doubled between 2000 and 2005; and now accounts for 1. 2% of American electricity consumption, though other estimates put the figure at 4% . Companies now spend as much as 10% of their technology budgets on energy, says Rakesh Kumar of Gartner, a consultancy. Power consumption has increased because of the rise of the internet, of course, but also because of way in which computers have historically been designed: to maximize performance at all costs. Between 1996 and 2006, the number of servers in use went from 6 million to 28 million and the average power consumption of each server grew from 150 watts to 400 watts. But things are now starting to change and the computer industry has been seized with enthusiasm for ' green computing'.