Problem: Given the complexity of datacenter design and operation, energy efficiency changes must be closely monitored for overall effect. You can upgrade items to more energy-efficient equivalents, but unless you look at the big picture and understand how the pieces fit together, you could end up being disappointed with the outcome.
Solution: PUE is suggested as the indicator of whether efficiency actually got better or worse. PUE = (Total Facility Power)/(IT Equipment Power)
PUE is a simple metric to get a big picture view of datacenter efficiency design and cost effectiveness. Without a metric like PUE, the authors suggest that the engineer could not measure the datacenter efficiency to see if it had improved.
Analysis: Total datacenter costs must be considered: servers, infrastructure, running costs (energy, management, etc). In 2001, the sum of infrastructure and energy costs was equal to the cost of a 1U server. In 2004, the infrastructure cost alone was equal to the cost of the server. In 2008, just the energy cost was equal to the cost of a server.
PUE is a simple metric that analyzes datacenter efficiency in terms of how much overhead there is for a given set of servers in the datacenter. However, PUE neglects an analysis of the actual servers in the datacenter. Work/Watt is an important metric that cannot be neglected. It is likely that servers can be more easily upgraded than datacenter infrastructure.
Conclusion: PUE is a useful metric to analyze the overhead beyond the servers but is not the only necessary metric. Work/Watt is important. The power used by computer equipment to accomplish the work/computation is the central datacenter energy efficiency issue.