The EPA expects approximately 25 percent of servers shipped to qualify for the Energy Star logo, he said.

The specification has been criticized by some for measuring idle consumption only, rather than the work done for a given amount of energy. The EPA has called it a useful first effort that gives customers a starting point to compare the efficiency of different systems.

The EPA says power supplies can vary considerably between server models, and that many servers still spend much of their time doing little or no work despite the wider use of virtualization.

The agency is working on a "Tier 2" specification that will aim to measure work done. Fanara said it hopes to publish it by Jan. 1 next year, although he acknowledged that timeframe is ambitious. The biggest challenge isn't a technical one but reaching consensus among the players involved, he said.

Critics have also noted that resellers may reconfigure systems in the channel, by adding more memory and disk drives, for example. The EPA will rein in such behavior as much as it can, but customers should check that the configuration they receive is the one that qualified for the program in the first place.