Exploring the concept of comparative PUE: is Power Usage Effectiveness a metric only suitable for comparing a data centre with itself?

Author:Bitterlin, Ian F.

The old adage about being able to improve only those things that you can measure has never been truer than in the data centre arena. As energy costs rise, carbon taxes become real, penalty charges loom and everybody wants to be seen to be 'green', the data centre has (rightly, as a large consumer of power) attracted the attention of the sustainability band-wagon. In a rush to be seen to be doing 'something' some trade organisations have produced metrics (measuring criteria) that can be applied to a data centre to prove that it is not wasteful of energy. If we ignore the sticky problem of 'what' the data centre actually 'does' we are left with one metric that has found almost universal favour--Power Usage Effectiveness, or PUE.

Defining PUE

The definition of PUE could not be simpler, or more open to interpretation.

PUE is simply the ratio of the total power being supplied to the whole data centre facility to the power consumed by the IT load. So it is always greater than unity and the closer to unity it gets the better the data centre is meant to be. Many people regard this as the power 'overhead' but, clearly, it makes no judgement as to the 'efficiency' of the IT load itself--a sacred cow of giant proportions.


Even the diagram above clouds the issue since it implies that 'B' is the 'microprocessor load'only, whereas in practice 'B' will always include the on-board power supplies and cooling fans of the IT 'box'.

The difference between the two power measurements includes such energy consumers as:

[] Fans and pumps associated with the mechanical cooling system

[] Compressor motors of chilled-water and other cooling plant

[] Heaters and humidifiers

[] Fans & cooling compressors associated with fresh-air make-up systems

[] UPS efficiency losses

[] Electrical distribution losses

[] NOC including Security and Building Management Systems

[] Internal & external lighting and cameras

However the losses are dominated by the first three on the list--i.e. the power consumption of the mechanical cooling plant. One of the reasons why the definition of PUE is often blurred is best demonstrated by an example:

Consider a small data centre (incl. NOC) consuming 800kW (900kVA @ 0.89 PF) located within a larger office complex that is separately consuming 720kW (900kVa @ 0.8PF). The total load for the one facility is therefore 1520kW (1800kVA @ 0.84PF). Inside the data centre the IT load consumes 445kW (445kVA @ 1.0PF). If the facility manager can only...

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