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The Power Usage Effectiveness (PUE) standard is one of the leading benchmarks for measuring energy efficiency in a data center. But there are some situations where an energy-efficient change to a data center will actually register as more wasteful by the PUE. For this reason, it’s important to know the gaps in the PUE metric.

Winston Saunders has described just such a case on the Intel Server Room Blog in an article titled, “Turning the Tide: CIO’s dilemma with PUE”.

The dilemma, according to Saunders, is this: “If improving the efficiency of your data center is an important goal, should you incentive [sic] the organization to improve PUE?”

Taken as a goal in and of itself, the PUE may lead data center operators astray. Take this example discussed by Saunders. A data center is running old servers with processors from 2006 (e.g. Intel Xeon 5160). These processors consume more power and perform less calculations than processors that came out in 2010 (e.g. Intel Xeon 5670).

Here’s the problem: if that data center chose to upgrade their old and inefficient servers to newer systems running more efficient and higher-performing processors, the PUE metric of that data center would actually increase (meaning, that data center would be less efficient according to the PUE).

The reason why this would happen is because PUE compares how much energy is consumed by IT equipment in a data center compared to all other energy expenditures (for lighting, cooling the servers, and other infrastructure). So, the more of the total energy is used to power the servers and not the other systems, the better the PUE score and – in theory – the more efficient the data center.

Back to our example of the data center that upgraded its old servers. This data center’s new servers drain less power. This means that the IT portion of total energy consumption in the data center decreases, which means the PUE will register this hardware upgrade as less energy efficient. Of course, the truth of the matter is the exact opposite.

Saunders illustrates the importance of considering PUE in context very well in his article. Anyone responsible for controlling data center costs should consider this (and other) gaps in PUE when making truly energy-efficient decisions.