This paper provides several metrics to characterize the efficiency of data centers. Power Usage Effectiveness (PUE) measures the fraction of the total facility power devoted to IT work. Compute Power Efficiency (CPE) measures the overall efficiency of the data center, considering both power and cooling and the utilization of the data center IT equipment. The paper describes several surveys characterizing PUE for enterprise data centers. Based on the data collected to-date, a PUE of 2.0 is expected for a world-class facility. In the near future, data center IT power consumption is likely vary more dramatically in response to workload changes as IT hardware power management technologies are adopted broadly. Data center power and cooling infrastructure must be designed with such variation in mind so that the overall efficiency is not impacted.

A simple data center infrastructure and energy cost model is presented to exposes the dramatic change in the relative importance of hardware costs to infrastructure and energy costs. Accurate models capturing these costs must be considered by organizations seeking leadership IT cost structures.