Power Usage Effectiveness � The Sequel

Julius Neudorfer

Like any popular movie, there is usually a sequel. It looks like the rise in popularity of the Power Usage Effectiveness (PUE) metric follows that same path. Clearly PUE has become the most popularly quoted data center metric lately, perhaps in some cases if only as a misguided marketing tool. This seems to be somewhat akin to the 'Mirror, Mirror on the wall, who is the Greenest Data Center of them All'-type of pronouncements recently used by some high-profile social media organizations.

On May 17th, the Data Center Efficiency Task Force issued a 14-page document entitled, 'Recommendations for Measuring and Reporting Version 2-Measuring PUE for Data Centers," or 'PUE 2.'

While PUE was originally created by The Green Grid in 2008, it was adopted in early 2010 by the Data Center Efficiency Task Force, a group consisting of 7x24 Exchange, ASHRAE, the Silicon Valley Leadership Group, U.S. Department of Energy Save Energy Now Program, U.S. Environmental Protection Agency's ENERGY STAR Program, United States Green Building Council, Uptime Institute and, of course, The Green Grid.

The updated PUE version is presumably meant to clarify and reiterate the updated measurement requirements of the PUE metric and presumably prevent "PUE envy." It reflects and reiterates the 'Harmonizing Global Metrics for Data Center Energy Efficiency' definition of PUE 'Power Usage Effectiveness,' which was released by the task force in February of this year.

While it is still called PUE, it now specifies energy usage (expressed in KWH) not power (KW) as the primary basis for the metric. This still seems to be one of the sources of confusion to those using and quoting PUE numbers for their data centers.

There are four PUE measurement categories (0-3) of which three upper categories (1-3) specify annualized energy consumption as the basis for the calculations. Even PUE "category 0," which still allows power demand expressed in KW, now requires that the calculation be based on the highest power used by the facility, not just the lowest power measurement taken on a cold night when the chillers are off. 

PUE Category 0: This is a demand based calculation representing the peak load during a 12-month measurement period.

This should help level the playing field of some seemingly exaggerated PUE claims.

The PUE 2 document is an enhancement and continues to follow the '3 Guiding Principles' and the 'Harmonizing Global Metrics' document issued in February by The Green Grid and the task force that was also adopted by some international organizations.

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