Speed and power have long been the most important criteria when judging a supercomputer's worth as a number-crunching lab workhorse, but energy efficiency is fast catching up. The greenest supercomputers are those that can process the most scientific calculations per second while drawing the least power.

During the first half of this year, the power of the world's fastest supercomputers increased in aggregate 15 percent, whereas average supercomputer energy efficiency rose 10 percent, according to the Green500 List, a project led by Virginia Tech computer scientists Wu-chun Feng and Kirk Cameron to rank supercomputer energy efficiency. "While the supercomputers on the Green500 are collectively consuming more power, they are using the power more efficiently than before," the organization said in a statement following the release of its fifth and latest ranking in June.

High-tech giants like Google and Microsoft are getting socked with high electric bills to cool ever-expanding data centers (computers don't like heat or humidity). In an attempt to lower costs, some of them are using fresh air to help maintain optimum temps–and now there's a way to gauge how much money and energy they can save by taking this tack. The Green Grid, a Portland, Ore.,–based consortium of 208 businesses (including Google and Microsoft) with megadata centers, today launched a free tool on its Web site designed to help U.S. and Canadian companies figure out how Mother Nature can help them keep their cool—and how much dough and energy they can save by tapping her reserves.

Potential savings depend on a number of factors, including where a data center is located, the local cost per kilowatt hour of power, how much energy a facility uses and the specific temperatures and humidity levels required, according to Mark Monroe, a member of the Green Grid's board of directors and Sun Microsystems's director of sustainable computing.

The computer you're reading this on may not seem like a huge energy waster, but the power consumption adds up when joined by the other PCs worldwide (Stamford, Conn., research firm Gartner estimates there are more than one billion). A study released last week puts a finer point on this assertion, reporting that U.S. workers waste $2.8 billion annually in energy costs by failing to shut off their PCs at the end of the work day. What's more, machines left on during off hours may emit up to 20 million tons of carbon dioxide (CO2) this year alone, roughly the equivalent impact of four million cars.

The 2009 PC Energy Report, (conducted by Harris Interactive and commissioned by London-based energy-management software maker 1E Ltd and the Alliance to Save Energy, a Washington, D.C., coalition of business, government and environmental organizations) says that nearly half of U.S. workers leave their PCs running overnight. Among reasons cited, according to the report: it takes too long to shut them down, people forget to turn them off or they deliberately leave them on so they can receive software updates overnight.