This note, the first of two to address data center power consumption metrics, defines and explains three popular metrics: the Power Usage Effectiveness (PUE), Data Center Infrastructure Efficiency (DCiE), and Data Center Productivity (DCP) ratios. The second note will define and discuss the basics of metrics intended for individual data center components, such as servers. Both types are necessary in the pursuit of a highly power-efficient IT operation. Understand these metrics, and learn how they can apply to daily data center operations.