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energy efficiency FACILITIES


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WITH CONCERNS over energy costs a perennial issue, and environmental sustainability only likely to become more important in the future, minimising power wastage is a priority not just for data centre operators but their customers also.


Power Usage Effectiveness (PUE) is an important and recognised metric for understanding how efficiently a data centre is delivering energy to the Information Technology (IT) equipment on which its services are based. Stated simply as the ratio of total energy consumed by a facility to the energy expended by its IT equipment, a PUE rating is easy to understand and, at first sight, easy to calculate. It gives a numerical rating for how much power, relative to that needed for its IT equipment, a data centre is expending on other functions, in particular cooling.


Many data-centre operators now quote their PUE ratings in marketing messages, so keen are they to exploit their efficient use of power as a measure of their worth. The closer to the figure of 1.0, the better is the data centre minimising power wastage. But with that new visibility comes concern that PUE ratings may not be a fair way to compare the energy efficiency, let alone the overall productivity or effectiveness, of one data centre to another. Is there a danger that the aggressive selling of impressive PUE ratings by some operators may reduce the credibility and therefore the usefulness of the metric as a whole?


30 www.dcsuk.info I February 2015


As always, the buyer must beware. Sometimes a quoted PUE rating raises more questions than its numerical simplicity appears to answer, especially if it purports to reach, or even exceed, the ideal but unrealistic limit of 1.0.


The PUE metric, first published in 2007, was developed by The Green Grid Association, a global non-profit, open industry consortium of end users, policy makers, technology providers, and utility companies working to improve the resource efficiency of information technology and data centres throughout the world. Since then, The Green Grid has continued to refine the metric measurement methodology with collaborative industry feedback


In fact the PUE metric was not originally intended as a means of comparing energy efficiency between different data centres. Rather it was devised as a tool to enable individual data centres to monitor their own efficiency over time, taking into account the inevitable changes in both equipment and operating practices that would occur during its operating life.


Given the eagerness of data centre operators to publicise PUE ratings they calculate to be lower than their competitors, the Green Grid has over time produced a set of rules and guidelines regarding the process that organisations should follow when making public claims. These go some way to helping


is in the details of PUE


PUE is a useful tool for measuring and monitoring a data centre’s efficient use of power but care must be taken when comparing figures across multiple facilities. By Victor Avelar, Director, Senior Research Analyst, Data Center Science Center, Schneider Electric.


decision makers judge how a particular PUE rating may have been calculated and what weight should be attached to it.


The Green Grid classifies publicly reported PUE results using four categories: Unrecognised, Reported, Registered, and Certified. The first two categories are essentially self-assessments by the reporting organisation which may claim that it followed the Green Grid’s measurement recommendations and nomenclature guidelines (Reported) or make no such claim at all (Unrecognised). The Green Grid makes no comment on results presented in either of these categories.


Organisations with Registered and Certified results have provided the Green Grid with additional data about their measurements so that the Green Grid can analyse overall industry performance and energy-efficiency trends over time. Only Certified results, which have been achieved using the most stringent data-reporting requirements, qualify for inclusion in any program created by the Green Grid to award or recognise data-centre efficiency.


The first element to consider therefore when considering a claimed PUE rating is what classification can be applied to the results. In the Unrecognised classification, a data centre operator may choose to ignore some or all of the measurement and calculation recommendations set out by The Green Grid.


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