Developed by The Green Grid consortium for efficient IT, PUE compares power going into a data centre to power used to run the centre's information technology. This means the perfectly efficient data center would have a PUE of 1, that is, all the incoming power goes entirely to driving the IT, with no energy wasted through heat, power conversion losses or other inefficiencies.

Other  Ways to Grade 'Green’ Data Centres 

For big companies like Google and eBay— and even for mid-market and smaller businesses — achieving a PUE that’s as close to 1.0 as possible is something of a holy grail, an aspiration that promises the right to brag about how green and energy-efficient they are. And SMEs that outsource their data centre operations are often marketed to at least partly on the basis of how efficient and low-PUE their service provider is.

Except PUE doesn’t really deliver a consistent picture of data centre efficiency. Data centre consultant Kevin Miller gives the example of one company whose PUE seemed to keep getting better and better, except that the measurement had nothing to do with improvements to the IT, but with the power used by standing air-conditioning units brought in to cool off hot spots in the data centre: “The worse the efficiency got, the better the published PUE as the in-room AC units just ramped up a little more.”

Another problem, highlighted by organisations like Greenpeace, is that a low PUE doesn’t necessarily indicate an environmentally friendly data centre. A very efficient facility, for example, could be 100-percent powered by coal, while a less efficient one runs entirely on solar power. PUE, ultimately, gives only part of the picture.

Which brings us to a newer way of measuring data centre “greenness”: the CUE (for “carbon usage effectiveness”). Also developed by The Green Grid, CUE was designed to give businesses a better picture of how sustainable — or not — their data centre operations really are.

When used in combination with the power usage effectiveness (PUE) metric, data centre operators can quickly assess the sustainability of their data centres, compare the results, and determine if any energy efficiency and/or sustainability improvements need to be made,” notes “Carbon Usage Effectiveness (CUE): A Green Grid Data Centre Sustainability Metric,” a 2010 whitepaper from The Green Grid. CUE builds on the PUE by factoring in the carbon emissions generated through data centre operations. It’s calculated by dividing a data centre's total energy-related carbon emissions by the power used to drive IT, or by multiplying PUE by the data centre's carbon emission factor (CEF), which is the amount of emitted carbon dioxide (or carbon dioxide-equivalent) in kilograms per kilowatt-hour.

(Other efficiency metrics include DCiE, for “data centre infrastructure efficiency,” and WUE, for “water usage effectiveness.”)

With all these different metrics still being tested, it’s clear we have a way to go yet before there’s any one measure that can accurately reflect a data centre's true, complete efficiency and environmental friendliness. The takeaway here for businesses that want to green their data centres — or to outsource operations to the most sustainable provider they can find — is to look at more than PUEs and CUEs, and to ask deeper questions about what the data centre is doing to minimise waste and maximise smart, efficient and low-impact use of resources

If your looking to gain a better understanding of your data centre facility, or want to know more about PUE, CUE or WUE and other metrics, contact us on 01993 774444 or email