The Green Grid paper on data center power efficiency metrics covers two key measures of data center infrastructure efficiency: PUE and DCiE (formerly DCE). The basic notion is that the most efficient facility is one in which most of the power goes directly to power the IT equipment, rather than to cooling, etc.

Simply put, PUE (Power Usage Effectiveness) is defined as data center power consumption divided by ICT equipment power consumption. (DCiE is the inverse of PUE.) Put another way, data center PUE = (ICT Power + non-ICT Power) / ICT Power. Over 70% of non-IT power goes to cooling in a typical facility.

Legacy PUE values are typically above 3.0. "Mega data centers" are claiming to be driving PUE down below 1.5.

Commentators have questioned the potential gaming of PUE numbers for PR purposes and the subtleties of using PUE to compare facilities. While these cautions are valid and useful, we should not lose sight of PUE's potential value to each individual facility seeking to improve its own performance.

Update 2010.02.22:
PUE got a boost this month from a group of eight US government agencies, industry consortia, and standards bodies. The Agreement on Guiding Principles for Energy Efficiency Metrics states, "Power Usage Effectiveness (PUE) using source energy consumption is the preferred energy efficiency metric for data centers."

Update 2010.03.10:
Digital Realty Trust's survey of ~300 North American data center managers reports that "The average reported PUE energy efficiency rating for respondents' data centers is 2.9; and one in six respondents report PUE ratings of less than 2.0 for their facilities."

Update 2010.04.12
PUE got another boost this month when US, European, and Japanese groups reached an agreement that "designated Power Usage Effectiveness (PUE) as the industry’s preferred energy efficiency metric." There is a little more detail in the Guiding Principles released earlier this year:

- Power Usage Effectiveness (PUE) using source energy is the preferred energy efficiency metric. PUE is a measurement of the total energy of the data center divided by the IT energy consumption
- The industry should improve the IT measurement capabilities to ultimately enable taking the measurement directly at the IT load (e.g. servers). At a minimum IT energy measurements should be measured at the output of the UPS
- For a dedicated data center, total energy measurement should include all energy sources at the point of utility handoff. For data centers in larger buildings, total energy should include all cooling, lighting, and support infrastructure, in addition to IT load

Update 2010.05.11
Google's Bill Weihl states that over 70% of non-ICT power consumption is for cooling.