Both Google and Microsoft in the past month have disclosed details of their data center energy usage, confirming that the two software giants operate some of the most efficient facilities in the world.On Oct. 1, Google said it is averaging a Power Usage Effectiveness (PUE) rating of 1.21 across its six data centers, while Microsoft said this week its testing shows that the 40-foot shipping containers packed with servers the company is installing in its new Chicago data center can deliver a PUE energy efficiency rating of 1.22, which may actually exceed the PUEs reported by Google since Microsoft includes house lighting and office loads in its PUE calculation. "They are required to run the facility, so we count them as overhead," Microsoft's Mike Manos said.

PUE is an emerging standard promoted by The Green Grid and others in the data center industry to provide a consistent way to measure the ratio of power delivered to IT equipment versus the total amount of power used by the facility. PUE allows data center managers to calculate how much power is driving the actual IT equipment versus non-IT elements such as cooling and lighting. The PUE metric is defined as the ratio of the total power consumed by a data center to the power consumed by the IT equipment that populates the facility. According to Data Center Knowledge, the typical enterprise data center is estimated to have a PUE of 2.0 or higher, which indicates that for every watt of IT power, an additional watt is consumed to cool and distribute power to the IT equipment.

The PUE standard has both supporters and detractors as a way to measure data center energy efficiency. "PUE is a compelling way to measure energy efficiency in data center facilities because it offers our industry an apples-to-apples comparison similar to the miles-per-gallon (MPG) fuel efficiency rating that the auto industry uses," according to Jim Smith, VP of engineering at Digital Realty Trust, the world's largest owner of data center properties. "Regardless of whether you are driving a pickup truck or a sports car, miles per gallon is a clear standard for measuring efficiency across all makes and models."

Jack Pouchet, director of energy initiatives with Emerson Network Power, faults the PUE standard for having several restrictions and limitations. For one, PUE doesn't take into account the time period measured. "Sure, if you take the reading at 9 AM while it is still cool outside and the chiller plant isn't running, the number can be very good. But go back in August at 1 PM to the same site. There's a huge difference. It's sort of like coasting downhill in your car and getting 200 MPG from your instantaneous reading meter. Then go to fill the car up a week later and find you only averaged 25 MPG for the week."

"PUE assumes the power going to the IT equipment is A) required, B) performing useful work, and C) that work has socio-economic value, not just some crazy videos or rants. This is far from the case as in most data centers: up to 30% of the servers are orphans (not used at all in the last 6 months) and of the remaining 70%, less than 10% are actually in service at any one time. One can quite easily make a case for a legacy data center in Miami, Fla., with a PUE of, let's say, 4 as being far more productive in terms of useful work than a bleeding edge San Jose-based data center with a PUE of 1.2," Pouchet elaborated. "PUE is a meaningless metric for comparing any two data centers."

"PUE is an 'interesting' aspect and nice to know, but using PUE to run a data center would be like trying to run the Indy 500 by looking in your rear-view mirror the entire race," Pouchet concluded.

The latest PUE measurements reveal that both Microsoft and Google are world beaters when it comes to data center energy efficiency. In a new section of its Web site dedicated to data center efficiency, Google adds this interesting metric: "In the time it takes to do a Google search, your own personal computer will use more energy than we will use to answer your query." If the Google searches you've been doing lately qualify as "useful work," then the latest PUE measurements might make you want to crown Google as the narrow winner in the data center energy efficiency race. Otherwise, you'll probably want to hold off waving the checkered flag.