While there continues to be a bit of difficulty in trying to quantify the energy efficiency of a data center’s IT systems, the efficiency measurement of the data center facility infrastructure is now is a well-defined metric. It is known as Power Usage Effectiveness (PUE), which was introduced by The Green Grid in 2008. The Green Grid is a global consortium of companies, government agencies, and educational institutions dedicated to advancing energy efficiency in data centers and business computing ecosystems.
PUE became an internationally agreed upon metric in 2011, when the U.S. Dept of Energy, EPA, European Union and Japan, agreed to it as a mutually satisfactory metric. This article is an part of the DCK Executive Guide to Energy Efficiency 
and the third article in a 5 part series on Energy Efficiency 
Understanding the PUE Metric – Power Usage Effectiveness
The basis of the PUE metric is relatively straightforward; it is the ratio of the Total Energy being used by the facility (including the IT Energy) divided by the Energy used by the IT Equipment. This is measured on an annualized basis. The range for PUE measurement is 1.0 (theoretically perfect – 100% efficient) with no upper limit (very inefficient). As can be seen by this example at a PUE of 2, the facility power and cooling systems have used as much energy as the IT equipment.
Typical Energy Use at a PUE of 2
Insofar as the physical data center facility and the related power and cooling infrastructure, the ever rising power requirements have strained many older data centers so that they are either incapable of fully supporting the higher power and higher density cooling loads of newer IT hardware, or they are just marginally supporting the loads, but are doing so very inefficiently.
Cooling: Room for Improvement
Cooling typically represents the majority of facility energy use and offers the greatest opportunity for improvement.
Previously, data centers were primarily focused on reliability, and not energy efficiency. On average, older data centers use twice as much energy for cooling, as was delivered to the computing equipment (this represents a PUE of 2.0 or 50% operating efficiency).
In some cases, the PUE of some older sites are even worse with a TE of 2.5 – 3.0. This meant that they were using far more energy for power and cooling systems used to support the computing equipment than the energy actually used by the IT equipment.
New mainstream data centers are being designed and built currently that typically have much better operating efficiencies, with a range of 1.3 – 1.6.
For the complete series on data center energy efficiency download the Data Center Knowledge Executive Data Center Energy Efficiency 
in a PDF format compliments of Digital Realty.
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URL to article: http://www.datacenterknowledge.com/archives/2012/08/28/energy-efficiency-measuring-facility-power-use/
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 DCK Executive Guide to Energy Efficiency : http://ads.madisonlogic.com/clk?pub=81&pgr=68&src=4712&ctg=1&tstamp=20120912T191634&ast=21023&cmp=2805&crv=0&frm=293&yld=0
 5 part series on Energy Efficiency: http://www.datacenterknowledge.com/archives/2012/08/14/dck-executive-guide-to-data-center-energy-efficiency/
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 Julius Neudorfer: http://www.datacenterknowledge.com/archives/author/juliusn/
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