It is not always clear what the right way is to measure the success of your data center's energy strategy.  While many would promote PUE as the right way to go, it does not tell the full story.  Think about these examples:

• A data center is commissioned, but no IT equipment is installed.  The PUE is infinitely high, but it doesn't mean the data center is inefficient.  As IT equipment is added, the PUE drops but the data center's efficiency did not improve.
• As servers and storage are consolidated or virtualized, the overall power consumption of the data center decreases.  Not only is there a reduction in the IT power, but a smaller corresponding reduction in the cooling required.  While the net is a win (more power capacity and less power usage), PUE typically increases.

These aren't the only bad examples of PUE, but they illustrate the fact that it is simply a ratio and it must be understood when talking about it. 

What I advocate is business metrics, which take into account the reason that the data center is there.  Metrics that correlate the work that the IT operation does to support the company, and correlate that work to the power being used.  While there are numerous attempts at doing this in a general context (such a the DCiE effort at The Green Grid), the real answer is that the metrics will be different for every organization.  If your company's business is contingent on your supply chain, the applications that you run and their performance are going to be quite different than a financial company whose business is dependent on frequent and instantaneous electronic trading.

I explored this topic in more depth in an article that I wrote for AFCOM, one of the leading data center operator organization.  The October 2009 Communique includes the article.

Read "How To Measure Success In The Data Center" at AFCOM.