Data centers use more than 1.5% of all electricity in China and the U.S., with continuing growth. This paper reviews the power hierarchy levels within modern data centers. It considers energy consumption and power electronics challenges across all levels of a data center, including building distribution, dc architectures, and conversion down to the board level. Power electronics plays a central role throughout the hierarchy, and emerging approaches are described. Strategies that enhance center energy efficiency, both in terms of overall center operation and in terms of computation performance, are discussed.