Rackspace has more than 20,000 servers, so even slight differences in energy efficiency could have a big effect on costs. For instance, if each server uses about 200 watts per hour -- and the energy consumption could be reduced by 2.5 percent, or 5 watts each -- that would save 100 kilowatts of energy. That's enough to power another 500 servers.

Workload isn't the only factor affecting a server's energy use. So does design. Some vendors put seven fans in their servers, others, five, said Froutan. "Do those five fans have to spin faster?" he asked.

Froutan was uncertain how much the efforts to devise a standardized measure of performance and energy use would help. The test workload might not exactly mimic the applications in his data center, he said. "Any data is better than no data," Froutan said, adding that he suspects he will still have to do is his own testing to supplement any vendor-supplied benchmark results.

Server makers now provide energy-use data, but they don't report that information uniformly. They may, for instance, offer an efficiency measurement of performance at 100 percent workload, which is not what customers typically use. The final draft protocol expected to be released by next week would measure performance per watt against a variety of workloads. The protocol's creators hope it will eventually be based on existing workload benchmarks developed by Standard Performance Evaluation Corp. and Transaction Processing Performance Council.

Bill Leo, CIO of Mercer Delta Consulting LLC in New York, said he doubts whether the effort to develop a benchmark will lead to energy improvements. "Unless they are going to be able to really compete with each other in this area, I don't know if it's going to have a whole lot of value," he said.