Does data centre uptime affect energy efficiency?


Does data centre uptime affect energy efficiency?

Mark Fontecchio, News Writer

The federal Environmental Protection Agency recently found that a data centre's uptime has no statistically significant effect on its energy efficiency. But does the claim hold water?

At first blush, it would seem that uptime would hurt data centre efficiency. Typically the more uptime a facility has, the more redundancy it has to build in to account for equipment failure. But that apparently is not as large a factor as other design elements.

"Tier level was not a huge predictor of energy performance," said Alexandra Sullivan, an engineer in the EPA's Energy Star program for commercial buildings. "When we looked at the data, we did not observe a significant relationship between tier and energy use."

The things that we've done around energy efficiency -- none of those were engineered for tier classification.

Tom Deaderick,
director of OnePartner's Advanced Technology and Applications Center

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The data was collected between March 2008 and June 2009. For the study, Energy Star looked at more than 100 data centres to determine their energy efficiency. The agency is using the information to create an Energy Star standard for data centre facilities, which will be released on June 7. The software will allow companies to rate their data centre's energy efficiency from 1 to 100, a scale similar to that for other commercial buildings.

The contention that a data centre's uptime and energy efficiency are mutually exclusive was not a surprise to Tom Deaderick, director of Tier 3-certified OnePartner's Advanced Technology and Applications Centre (ATAC), a hosting centre in southwest Virginia. He said there is no reason why designing a facility for high availability necessarily hurts energy efficiency. A good design can achieve both, he maintains

"The things that we've done around energy efficiency -- none of those were engineered for tier classification," he said. "I really don't think the tier standards have a whole lot to do with energy efficiency."

The ATAC facility has energy efficiency designed throughout. Hot/cold aisles, blanking panels, perforated ceiling tiles over the hot aisles to exhaust hot air faster, grommets to further prevent wasted cooling air and neat cabling in the subfloor are all designs in the ATAC data centre to make it more energy efficient.

That said, the facility also has plenty of redundancy. It sits in a room that Liebert thinks only requires one computer room air conditioner (CRAC), but ATAC installed three to help ensure high uptime.

Other factors aside from redundancy have a much larger effect on a data centre's energy efficiency, which is often quantified by its power usage effectiveness, or PUE, which improves as it decreases toward 1. Capacity is one.

"There could be a Tier 2 site with a very light load, where they have a whole bunch of excess capacity and the PUE will be high," said Pitt Turner, executive director of the Uptime Institute. "Then there could be a Tier 4 site that has a load that is very close to full capacity, and its PUE is lower. The idle capacity is the biggest contributor."

Sometimes it can be as simple as the kind of equipment one uses, which Turner said was the second-biggest contributor to a site's PUE. For example, a Tier 2 site using outdated uninterruptible power supply (UPS) units running at 85% efficiency could have a higher PUE than a Tier 4 facility using UPSes with 95% or more efficiency.

"There are probably some pieces of the PUE calculation that could be attributed to the level of redundancy," Turner said. "But what we have found is that it is overwhelmed by partial load conditions and idle capacity."

Mark Fontecchio can be reached at