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energy efficiency FACILITIES

For example, the Green Grid awards no credits or percentage points for generating energy or recovering waste heat. An operator that generates some or all of its electricity requirements using renewable resources such as solar or wind energy may be making a valuable contribution to environmental sustainability, as well as creating opportunities for long-term cost reduction for itself and its customers, but these are irrelevant to the calculation of PUE as recommended by the Green Grid.

Another element to consider when comparing PUE ratings is how the various electrical loads in a data centre operation have been assigned in their calculation. The apparently simple separation between IT and non-IT equipment, essential for an accurate calculation, is in practice rather complex.

The Green Grid produces a comprehensive list of which equipment should be categorised as IT-only, and therefore appearing below the line of the equation used to determine PUE, and which equipment counts as ancillary, to appear above the line. In most cases, the classification is straightforward: computing equipment including servers, processing elements, storage arrays, networking equipment, and KVM (keyboard/video/ mouse) switches go below the line as do, monitors and workstations or notebook

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computers used in the network operations centre (NOC).

Along with these on the top line of the equation goes all other energy-consuming equipment needed in the data centre. This includes: cooling equipment such as chillers, cooling towers, pumps and air-conditioning units; lighting, and power distribution equipment including UPS (uninterruptible power supply) systems, switchgear, generators, power distribution units, and batteries.

However as technology evolves, the demarcation lines between the various types of equipment become blurred. Some cooling elements such as pumps, refrigeration, blowers and heat exchangers are now to be found within the IT equipment itself. To which part of the equation should the power associated with them be assigned? Maintaining consistency of calculation is an ongoing challenge for the Green Grid.

As well as continuously updating its classifications of what does and does not constitute IT equipment, the Green Grid also recommends the use of energy-measuring techniques throughout the data centre so that accurate calculations can be made. Measuring the total power expended by a facility at the utility meter is simple and inexpensive but does not make for accurate assignment of power losses for the purpose

of PUE calculation. Installing measurement apparatus throughout the data centre can be expensive, and introduces a further layer of complexity into the calculation of results which must also be taken into consideration.

When taking into account all of the inevitable energy losses in a data centre between cooling, lighting and measurement as well as running the IT equipment itself, it is impossible, following the recommendations of the Green Grid, for a Certified data centre to attain a PUE rating of 1.0.


PUE is a useful and widely accepted tool for measuring the energy efficiency of a data centre. However, it is best applied to monitoring the energy efficiency over time of an individual data centre as its contents and purpose evolve to accommodate changes in technology and customer demands. If used as a means of comparing one data centre to another, which may have a different configuration, a different purpose and located in a different environment, care must be taken to analyse how its rating was calculated and whether one is really comparing apples with apples.

£ Schneider Electric White Paper #158, Guidance for Calculation of Efficiency (PUE) in Data Centers, is available for free download at:

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