Design/facilities
Let’s Keep it Focused on Data Centre Efficiency
People are increasingly familiar with power usage effectiveness (PUE) - in part as a result of valid efforts from bodies such as a the Green Grid. However, Jim Smith, CTO at Digital Realty Trust, identifies that this increased recognition also results from many companies using PUE as a tool to enhance their marketing and PR initiatives. He argues that this can lead to confusion as to what PUE is really all about.
PUE is a metric, created by members of the Green Grid, which can be used to determine the energy efficiency of a data centre. It is calculated by dividing the amount of power which is used in a data centre by the power used to actually run the computer infrastructure within it – in essence PUE is the mechanical load in relation to the IT load. The figure a company presents as its PUE is therefore a representation of how effectively its data centre uses the power put into it, with efficiency improving as the ratio moves closer towards 1.
As of 6th April 2010, a number of significant organisations and legislative programmes have officially adopted PUE as their standard metric of choice for measuring and reporting data centre efficiency. In the US, the Environmental Protection Agency’s (EPA) ENERGY STAR programme and several federal energy management programmes are making use of the metric. The Department of Energy has also introduced PUE to its ‘Save Now’ programme. In Europe, PUE has
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been adopted into the European Code of Conduct for Data Centres, and Japan’s Ministry of Economy, Trade and Industry (METI) has introduced the metric into its green IT initiative and Green IT Promotion Council (GIPC). Yet despite the widespread usage of PUE, its core value remains its use as a tool for internal measurement and improvement.
While PUE has provided the industry with a common language in terms of how we speak about energy efficiency in the data centre, there is no absolute standard as to how PUE is advertised or talked about. Nor is there true consistency in how organisations are calculating it. As a consequence, PUE is a number which can be used in numerous ways. There are a lot of companies advertising low PUEs, but despite the ‘common language’, they are frequently referring to
different interpretations. When a company boasts a PUE of 1.2, you have to ask - are they taking the annual average, the annual peak, or perhaps the PUE at the time of commissioning of the facility? You don’t necessarily get to see all this when making a face-value comparison.
This isn’t to say there is a deceit happening – it’s understandable that an organisation wants to use the most favourable measurement available. Where it can be
February/March 2011
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