The more than 1 billion personal computers in the world use a tremendous amount of energy [source: Chapman]. Information technology companies, taken together, produce more greenhouse gas emissions than the airline industry [source: Keller]. But which computers are the most energy efficient? It depends what you're looking for.
In July 2009, Green500 compiled a list in which IBM had 18 of the 20 most energy efficient computers. The best among them: Warsaw University's IBM BladeCenter QS22 supercomputer cluster, which can, with one watt of energy, compute 536.24 million floating-point operations per second [source: Burt]. (When talking about supercomputers, we're really dealing with clusters -- many machines that work quickly and in tandem, working so well that they can be considered a single machine.)
The average consumer is unlikely to use a supercomputer -- which do anything from complex mathematical computations to modeling the universe after the Big Bang -- so which personal computers are the most green? In a June 2009 test, CNet found that Apple's 20-inch iMac was the most energy efficient all-in-one desktop, with an estimated annual energy cost of $12.78 [source: Brown]. The iMac, along with the other computers tested, qualifies for Energy Star Level-5 certification.
As for laptops, the crown for most energy efficient is always shifting, but look for that Energy Star certification. The environmental group Greenpeace has given credit to Apple, Acer and Toshiba for cutting down on harmful materials like brominated flame retardants and PVC plastics.
For more information about green gadgets and other eco-conscious topics, look over the links on the next page.