An Energy Star rating for servers arrives on Monday. And, like a lot of things in the technology world, the first version has some flaws.

The problem, say experts, is that it rates servers on how much energy they consume when idle, not when processing applications. That's sort of like rating a car's gas mileage by how much fuel it burns at a stop sign.

"It is a measure of efficiency, but [idling] is becoming less of an issue," said Jon Haas, director of the Eco-Technology Program office in Intel's Digital Enterprise Group, diplomatically. Servers in data centers that heavily invest in virtualization software – which let  a single server do the work of ten – don't experience much idle time. Thus, the standard doesn't accurately map to them.

Subodh Bapat, the vice president at Sun Microsystems who heads up the company 's energy and environmental efforts, and other IT execs have made similar complaints. Energy Star is voluntary, but it slips into becoming a de facto standard, Bapat noted in January. Many government agencies list Energy Star ratings as a mandatory purchasing requirement. Since government agencies buy so many servers, hardware vendors are often tempted to try to meet the standard in a wide variety of products.

The second version of the standard will try to take into account how the server performs under different types of workloads and it is arleady being developed. It won't be easy considering the diversity of data centers and applications, but vendors and customers ultimately would like to develop somewhat handy operation per watt standards.

"What we all want to get to is a miles per gallon metric," said Haas.

Although it isn't a comprehensive rating, the new Energy Star for servers could goose sales. "We will see rebates [from utilities] once it comes out," Haas said. Rebates are arleady playing a crucial role in data center remodeling. Both Yahoo and NetApp built million dollar plus data centers that were primarily paid for with rebates from PG&E.

And speaking of rebates, Haas told us to pay close attention to the rebates some utilities, such as Xcel Energy, are giving corporate users for replacing their desktops with thin clients. The Regional Transportation District of Denver, for instance, is converting 800 desktops to thin clients that are being paid for in part through Xcel rebates. The agency says it expects to net a savings of $600,000 over eight years in lower maintenance and power costs.

Haas, though, notes that corporations that move from desktops to thin clients have to bulk up on servers. Servers draw power and, because they are in data centers, they have to be cooled with air conditioning. "People are going to realize that thin clients have their limitations."

Intel, of course, derives a substantial portion of its revenue from chips for desktops and notebooks, which thin clients could erode. At the same time, air conditioning bills remain a big problem for data centrers, so this will be an interesting debate to watch.