The long-in-the-works Energy Star rating for data centers will be based on Energy Usage Effectiveness, a metric researchers at the U.S. Environmental Protection Agency created in the process of developing a rating system for mission critical facilities that already exists for other commercial buildings. Unlike the commonly used Power Usage Effectiveness metric, EUE is based on energy rather than power.

EPA will use a data center’s EUE, normalized for a variety of the facility’s characteristics, to calculate the 1-100 rating for that facility. Data centers in the top 25 percentile (ones that receive a 75 or higher rating) will qualify for Energy Star certification, EPA officials said in a conference call Tuesday.

“We are not trying to introduce EUE as a new term to the dialogue,” EPA’s Aleksandra Sullivan said. She leads the rating’s technical development for the agency. While metrics like PUE and DCiE (Data Center infrastructure Efficiency) have gained most industry acceptance, some still question their effectiveness and new efficiency metrics are in the works.

EUE is calculated by dividing total source energy by total UPS energy. The EPA is using source energy instead of site energy to be able to account for a variety of fuel types a facility uses in its energy efficiency equation. Those fuel types may include things like chilled water delivered from the utility. Source energy also accounts for losses that result from conversion and transport of energy to the site.

While the agency has not settled on which normalization factors will be used to derive a 1-100 rating from EUE, information on factors that were most likely to be included and those that were not likely to be included was presented.

Facility characteristics likely to be included are UPS energy intensity (UPS energy per square foot), UPS energy, the facility’s square footage and its tier level. While the EPA uses tier levels as defined by the Uptime Insitute, applicants will not be required to have the institute’s certification of their facilities’ tier level if it does become a normalization factor.

Factors likely to be omitted from the equation are heating and cooling degree days, data center type (traditional, hosting, Internet, etc.) and UPS utilization.

The EPA is developing the rating system based on energy use data over a 12-month period it collected from 121 data centers. Researchers were surprised to discover that outside temperature had little to do with a data center’s EUE.

They began the project fully expecting a facility’s location and the time of year to exert a strong influence on its figures. After observing the opposite, they decided that the number of cooling or heating degree days was not likely to make it into the equation.

Another surprising finding was that the presence or absence of airside economizers at a facility had little influence on its EUE figure as well.

The agency plans to launch the new rating system in April of 2010. Until then, it will collect more feedback from the industry, including another update session in November to present additional results.