Published on 4th December 2014 by Mark Monroe

The recent burst of stories about the NGD data center in Wales, UK, has stirred the world of data center efficiency experts by publishing their PUE as 1.0. There’s the article “Next Generation Data Claims PUE Of 1.0 In Wales,” for example, or “When Someone Claims PUE of 1.0, Is It Greenwash?” Also, Ian Bitterlin’s, “They Say that There Is No Such Thing as Bad Publicity - but 'They' Could Be Wrong...” Well, it seemed like I should jump in, too.

The controversy comes from the fact that PUE is defined as the total energy used by the data center divided by the energy used by the IT equipment. From conservation of energy laws in Physics, and from the definition of PUE, we know that the ideal value is 1.0, but that it can never be achieved in a world with friction, entropy, and the 2nd Law of Thermodynamics. Thus, we PUE-geeks know NGD cannot be telling the truth when they claim a PUE of 1.0. Not unless their water flows uphill and they have a perpetual energy machine in the generator yard.

Wrong By Definition

PUE is defined by The Green Grid to explicitly exclude in-facility generation sources from the energy use calculations, unless the source energy is counted in the numerator of the fraction.[i] NGD could claim the energy generated by the solar panels, but they would have to count all the incident solar energy as part of the total energy consumed. Since solar panels are typically 16-18% efficient in converting light to electricity, the resulting PUE calculation would be about 2.0, not 1.0

It would be similar to a bank claiming to give you 105% interest on your money, because they gave you back your original deposit. Or Liverpool claiming it really tied Leicester City last week, because the own goal scored by Simon Mignolet didn’t count in the net scoring.

What I Meant to Say Was…

What NGD means in their claim is that they believe their solar array will offset the overhead energy used by their building. They claim the 4,000 panel photovoltaic (PV) solar array will produce about 1M kWh of energy per year. I coined the term “Net Zero Overhead” (or NZO) for this type of arrangement in a blog back in May 2013. NZO does a better job of describing what is going on than does PUE 1.0. In essence, NGD claims they have canceled out the impact of their overhead systems by adding back an equivalent amount of energy to the grid through their solar array.

Still Wrong In Their Claim

What’s unfortunate is that NGD must still be wrong, even if they claimed to be a Net Zero Overhead data center. Similar to Lifeline Data Centers in Indianapolis, who claimed in Aug 2013 to be “largely powered by solar energy,”[ii] NGD cannot be getting a significant amount of their energy from the solar array.

NGD claims to have 180MW of power available, and to have a conventional PUE of 1.18. At full load, this means 180MW x 0.18 = 32.4MW of overhead energy will be used at full load, 24 hours x 365 days per year. That’s 280M kWh per year. In the announcement article, NGD said they expect to produce about 1M kWh per year from the 4000-panel array, which is 0.35% of the overhead energy used.

The 1M kWh that NGD estimates the array will produce would offset about 114kW of constant load, enough to supply the energy for 3 x 30 Ton CRAC units running 24 hours a day.  

That leaves NGD open to further issues in that they cannot substantiate even their impossible claim. It’s even more unfortunate that customers appear to like the claim, even though it cannot be anywhere close to the truth.


[i]    Avelar, V., Et Al. "PUE™: A Comprehensive Examination of the Metric." The Green Grid, 2 Oct. 2012. P58.

[ii] Carroll, Alex. "Lifeline Data Centers Announces Four Megawatt Solar Array at Indianapolis Colocation Campus." Lifeline Data Centers, 26 Aug. 2013. Web. <>.



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Mark Monroe is Chief Technology Officer (CTO) and VP of DLB Associates. An expert in corporate sustainability, data center efficiency, and many aspects of Information Technology (IT), he has over 30 years experience in the IT industry, including responsibility for internal, customer-facing, and ... More

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