Data center electrical efficiency is rarely planned or managed. The unfortunate result is that most data centers waste substantial amounts of electricity. Today it is both possible and prudent to plan, measure, and improve data center efficiency. In addition to reducing electrical consumption, efficiency improvements can gain users higher IT power densities and the ability to install more IT equipment in a given installation. This paper explains how data center efficiency can be measured, evaluated, and modeled, including a comparison of the benefits of periodic assessment vs. continuous monitoring.