Harmonizing the PUE Metric

Julius Neudorfer
While singing in the green choir, apparently not everyone has been singing from the same page of the power usage efficiency (PUE) hymnal. A global task force was formed two years ago to deal with the apparent lack of harmony. The result, "Harmonizing Global Metrics for Data Center Energy Efficiency," is a 12-page document from The Green Grid that should have the U.S., the European Union and Japan presumably all singing together on key. 

The key organizations involved in the task force are: the U.S. Department of Energy's Save Energy Now and Federal Energy Management Programs; U.S. Environmental Protection Agency's ENERGY STAR Program; European Commission Joint Research Center Data Centers Code of Conduct; Japan's Ministry of Economy, Trade and Industry; Japan's Green IT Promotion Council; and, of course, The Green Grid.

This new release is the follow-up and continuation of the 'Three Guiding Principles' document released last January.

One of the more interesting points are that while the document provides some additional clarity and uniformity of methodologies to implement the PUE metric, it also indicates that one of the task force's future goals is to work on quantifying a method and metric to measure IT productivity. This has long been the 'missing link' in the data center efficiency model, since 'actual IT work output' is very nebulous and difficult to quantify to everyone's satisfaction.

There are several desired outcomes of the task force's activities:

Outcome (1)-Effective energy efficiency metrics that measure the actual IT work output of the data center compared to actual energy consumption. It is of note that in the process to define IT work output, the following interim measurements are being defined and/or validated.

Outcome (1a)-IT-Measure the potential IT work output compared to expected energy consumption and measure operational utilization of IT equipment.

Outcome (1b)-Data center facility and infrastructure-Measure the data center infrastructure and efficiency (Power Usage Effectiveness-PUE).

Outcome (2)-Measure renewable energy technologies and reuse of energy to reduce carbon. 

The new harmonizing document specifically excludes using any reuse of waste heat for other purposes (such as heating adjacent buildings), to offset or lower the PUE calculation. However, it does commend reuse of waste heat as a beneficial practice, but doesn't allow it in the calculation. Nevertheless, it does indicate that it is being addressed in a related, but separate, metric "Energy Reuse Effectiveness" (ERE) document, which was released in September 2010 by The Green Grid. 

The Bottom Line

This global harmonization effort should help bring consistency and further validity to the PUE metric, as opposed to the original PUE snapshot of power (KW) versus energy, expressed in KWH. The original PUE metric created universal energy efficiency awareness, and its inherent simplicity, while criticized as imperfect, allowed it to be easily understood and therefore used by many organizations. Perhaps now we will see meaningful PUE numbers, rather than artificially low claims by the marketing departments of data center operators and even some equipment manufacturers.

So to paraphrase the old Coca-Cola commercial, we can all sing 'green' together in global harmony.

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