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Understanding Data Centre PUE

To PUE or not to PUE (That is the Question!) What is it and why is it useful?

Date: 20th August 2015
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For a few years now the data centre industry has been moving towards a standard measure of effectiveness for the use of power within data centres.  Power Usage Effectiveness (PUE), whilst not yet a globally agreed measure is fast becoming the standard and formalising the calculation is well underway. But what is it and why should you bother?

PUE allows organisations and enterprises to gather data, measure and report the effectiveness of their data centre, or indeed data room, in terms of power use.

In simple terms, PUE is,

Total Energy Used in the Facility
Energy Used by the IT Equipment

For smaller organisations or facilities that are located within a larger mixed-use space, Partial PUE (pPUE) can be used. This allows the data centre manager to measure PUE within a set boundary such as a room or building, or an area such as the equipment owned by certain customers or departments.  For example, you may want to measure the pPUE of individual data halls, rather than the facility as a whole.

The difficulty for many businesses is the availability of the data and information required. Whilst assumptions can be made and data captured at different points it does mean that comparing PUE or pPUE across market sectors is difficult. However the real value of PUE isn’t the creation of a global league table and winners awards, but more importantly PUE gives anyone with one or more data centres or data rooms the ability to measure and compare the effective use of power over time and across the business. (There are a wide range of intelligent monitoring solutions available for data centres, you can see some of the ones we offer by clicking on the link and keep an eye out for our upcoming blog post on the “5 things you need to know about DCIM”)

PUE gives everyone the ability to measure and improve the use of power in their data centres.

So what should be included in each group for the calculation?
Total Energy Used by the Facility

  • Lighting,
  • Power for generators,
  • Switchgear,
  • UPS units,
  • Rack and power distribution units,
  • Air conditioning units,
  • Fans,
  • Heaters,
  • Pumps and dehumidifiers,
  • Fire and security systems and sensors and so on.

Plus the IT equipment listed below

Energy Used by the IT Equipment

  • Computers and servers,
  • Storage,
  • Monitors,
  • Laptops and any peripherals such as printers,
  • External drives,
  • Power USB hubs,
  • Routers,
  • Telecoms hubs and distributors and so on.

Whilst the Total Energy Used by the Facility can often be gathered at source from the utility meter supplying the facility, or a meter just prior to the data room (for pPUE), the data collected regarding the Energy Used by the IT Equipment will be more accurate the closer the source of the information is to the individual units that consume the power. For example, gathering power usage from the installed UPS units is not as accurate as the information available from the subsequent Power Distribution Units (PDUs), which in turn is not as accurate as the data gathered from individual meters immediately prior to the IT Equipment.

PUE and pPUE account for this within the standard definition by allowing 3 variations, PUE1, 2 and 3, with PUE3 being the most accurate. There is also three different reporting frequencies; Yearly (Y), Monthly (M) or Weekly (W) with each denoting what period the data has been averaged over. On top of this you then have the data collection frequency; Monthly, Weekly, Daily or Continuously. So a PUEL3YC (which is the standard measurement we use at Keysource) would be a measurement from PDU level on a continuous basis. 

World Class PUE

Given the lack of consistency in measurements and environments it is not valid to quote a ‘world class PUE’, however it is generally accepted that a PUE of 2 or less is considered good and less than 1.4 is considered very good. A PUE of 1 means that 100% of the energy is used by the IT Equipment and therefore the physical data centre is 100% efficient, a PUE of 2 or less would mean that 50% or more of the power used by the data centre is used by the IT Equipment and so on.

There are so many factors that effect the efficiency of data centres that “snap shot” PUE figures can be misleading.  For example:

  • Environmental conditions – in the winter (in the UK especially) a data centre will typically need less cooling therefore the power draw will be less – improving the PUE
  • IT load – as the IT load fluctuates over time the ratio of power used by the facility vs the IT load also changes, effecting the PUE ratio.

Because of this, continuous reporting and monitoring is the recommended way of tracking PUE, by not only Keysource but also The Green Grid; who developed the metric in the first place.

As touched upon it is also important to take into account the different environmental conditions of different regions. As such, PUE levels cannot be compared across regions because a higher PUE in one region might be relatively better than a PUE in another region but we will look at this in more detail in another post.

Remember, the key is consistency, define your measure, data sources and frequencies and stick with them. 

Design PUE or the PUE of new facilities

When a customer wanting a new facility designed and/or built approaches Keysource, they often have an idea of the PUE they want to achieve and a design is developed to meet this business need whilst also meeting the technical requirements.  However as this is a theoretical figure (admittedly based on some very complex mathematics!), how can you, as the customer, be sure that this is what you will be able to achieve and what if anything should you be aware of?  Find out more in our next blog looking at PUE.

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