Beyond the hype
While PUE is useful for measuring how power usage at an individual datacenter changes over time, public claims of incredibly low PUE ratings can't always be believed.
According to Brill, one company claimed to have a PUE of 0.9, which is impossible because that would mean its servers use more power than is actually delivered to them. The company said its use of solar power and windmills shouldn't count toward power usage, Brill says. Some companies might skew ratings by measuring UPS but not other power sources, or by measuring only during the cooler portions of the year, rather than calculating an average across 12 months, he says.
Meanwhile, Google has claimed an average PUE of 1.21 across six datacenters, and the most efficient of the six had an annual rating of 1.15 and a best quarterly rating of 1.13. If the numbers are accurate, only about two-tenths of a watt (for every watt of IT power) is needed to cool Google's servers and distribute power to IT equipment.
But how meaningful are Google's measures to your standard enterprise with just a few datacenters? If one Google datacenter fails, it has many others to pick up the slack. But if a company has a datacenter that must absolutely remain online at all times, lowering the PUE might require sacrificing redundant power supplies that maintain required uptime, Brill says.
Enterprises that want to measure PUE or analyze the energy consumption of individual devices have several options. Vendors like GDCM, Server Technology and Sentilla offer software products that measure and analyze power usage in the datacenter. Such tools can help reorganize IT loads to maximize power efficiency, or use chargeback to bill departments for their energy usage.
PUE, if not perfect, helps companies understand how IT load changes affect power usage, says Calvin Nicholson, director of product marketing for Server Technology.
"I think there's a lot of hype [around PUE]," he says. "But it's simple, it's easy for people to understand, and it makes sense."
Network World is an InfoWorld affiliate