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After more than one year of work across three countries, one of the taskforces of The Green Grid has finalized international guidelines for defining and measuring Power Usage Effectiveness. The purpose of these guidelines is to communicate data center energy efficiency across the borders of US, EU and Japan.

Dan Azevedo, one of the organization’s board members, said the taskforce for global harmonization of metrics felt it had dotted the ‘i’s on PUE. "We feel that we’ve tackled the problem pretty well," he said.

"Regardless of data centers’ design type, you now have equitable way to calculate [PUE], whether you’re in the US, the EU or Japan. So, there doesn’t seem to be any more open items."

There are multiple taskforces that work on the metric. One of them is a US regional task force, whose guidelines for measuring it are somewhat different from guidelines produced by the harmonization group.

Azevedo said the regional guidelines, while producing more accurate results, are meant for use within the region they apply for. The international guidelines produced by the harmonization group account for the differences between regions, such as the effect elevation has on efficiency of producing energy from natural gas.

"Every level that you go up to harmonize, you lose a little bit of accuracy from the regional step to global step," Azevedo said.

The main difference in the calculation is what the group calls "harmonized factors". The total-energy side of the PUE formula is multiplied by a weighting factor in the harmonized version to account for differences between source fuels or use of district chilled water. All of the factors are normalized for electricity.

Here they are:

The group’s other recommendations that apply to the international metric are: using energy as a unit of measure, measuring PUE continuously, measuring total energy at the point where utility hands the power off to the data center and measuring IT power at the output of the PDU.

The harmonized guidelines propose ways to account for on-site generation, as well as co-generation and the fuel purchased for that purpose.

For a detailed look at the guidelines download the taskforce’s whitepaper on the subject.

Now that the PUE harmonization is finalized, next steps for the taskforce include focusing on ways to measure IT work output, and utilization of IT equipment, as well as accounting for renewable energy technologies and waste-energy reuse.