Proving that size isn't everything, the Top500 List of supercomputers for the first time is looking at power efficiency.

With the IT industry increasingly looking at growing electric bills and calls for greener machines, the world's most powerful and largest systems are under scrutiny. The latest edition of the twice yearly Top500 List of supercomputers was unveiled Wednesday at the International Supercomputing Conference in Dresden, Germany. At the same time, the list authors also provided energy efficiency calculations for many of the computers on the list.

"Power consumption is becoming one of the most important aspects of computing," said Jack Dongarra, a co-creator of the Top 500 List and a distinguished professor at the University of Tennessee. "It will be the most important driving force for supercomputing in the future. Without focusing on that, building bigger machines will be prohibitive. We're trying to understand which machines are more efficient, why they're more efficient and understand the trends in high-performance computing."

IBM's Roadrunner supercomputer, which just last month became the first machine to break through the petaflop barrier, is not only the most powerful machine on the list, it's also the power efficient, according to Dongarra. The 3-megawatt system still uses enough energy to power an average-size shopping mall. Both Dongarra and Erich Strohmaier, an author of the Top500 List and a computer scientist at Lawrence Berkeley National Laboratory, credit the fact that Roadrunner is a hybrid machine - running both AMD Opteron processors and Cell chips - with its power efficiency.

"Roadrunner is the most powerful [system] and uses some of the most efficient technology," said Strohmaier in an phone interview from Dresden. "Comparatively, two smaller systems on the list use Cell blades and use less power because they are smaller. But Roadrunner is right up there."

As the industry now moves beyond the petaflop barrier toward the loftier exaflop barrier, Dongarra pointed out that increasing power efficiency becomes even more important.

"The projections for exaflop machines [show that they] will require on the order of 100 megawatts of power," said Dongarra. "It will require very special facilities to run that. The cost of running a petaflop machine at a modest electrical price will almost equal the money you pay for the computer."

He noted that engineers at the Oak Ridge National Laboratory are working with Cray Inc. to build two supercomputers -- Jaguar and Kraken. Both are in the early stages of production but are expected to surpass a petaflop. Dongarra said when they are fully functional, which is expected to happen in 2012, the power costs for both will total an estimated US$32 million a year.

Dan Olds, principal analyst with the Gabriel Consulting Group, said he has high hopes that advances in technology will drive down the cost of running supercomputers before the industry reaches the exaflop level, which isn't expected until to come for about 11 years.

"The problem is that these calculations just extrapolate power usage today onto future big systems," he added. "It doesn't take into account the advances in system and chip design that will lower energy usage on these upcoming mega systems. As the machines get bigger and more power hungry, technological development keeps pace to make possible what only a few years earlier was supposedly impossible."

Power concerns will be part of the equation when building new supercomputers but it won't be the main concern, said Olds. That top concern will remain performance.

Dongarra noted that Roadrunner runs at 488 megaflops per watt. BlueGene/P, another big hitter on the Top500 List, runs at 376 megaflops per watt.