The Green Grid has recently updated its PUE metric (Power Usage Effectiveness) that attempts to wrangle some of the uncertainty in the prior definition of the measure. Ted Samson has a nice analysis
One of the greatest strengths of the PUE metric, the industry standard for measuring data center energy efficiency, is its simplicity: Calculate how much energy your data center is consuming overall, then divide that number by how much energy your IT equipment alone consumes.
…At the same time, the simplicity has its shortcomings. For example, it gives operators much flexibility as to where to measure consumption — at the PDU or at the point of connection of IT devices — as well as how often to take measurements.
…In an effort to overcome this drawback, The Green Grid has unveiled four categories of PUE, ranging from Category 0 to Category 3, in a new white paper, “Recommendations for Measuring and Reporting Overall Data Center Efficiency” [PDF]. With each level, the measurements become more granular and the results more precise. Thus, a data center operator may choose to go with Category 0, which requires the least effort and fewest resources — but then those results won’t be viewed in the same light as a rival’s Category 3 PUE figure.
More in Samson’s article; you can also read what The Green Grid itself has to say about the topic in the related whitepaper.