PUE

PUE is a way of measuring efficiency of your data centre in taking into account your energy expent in computers and the total of energy spent over the facilities, the closest to 1.0 the more your energy is invested only in your servers:

PUE = Total energy / energy consumed by servers[1]

In October 2008, Google's Data center was noted to have a ratio of 1.21 PUE, Right behind Google, was Microsoft, which had another notable PUE ratio of 1.22.

Figure 1 : PUE diagram

References

[1]Margaret Rouse. (2009). Power Usage Effectiveness. Available: http://searchdatacenter.techtarget.com/definition/power-usage-effectiveness-PUE. Last accessed April 2015.