Data centers are a significant and growing component of electricity demand in the United States. This paper presents a bottom up model that can be used to estimate total data center electricity demand within a region as well as the potential electricity savings associated with energy efficiency improvements. The model is applied to estimate 2008 U.S. data center electricity demand and the technical potential for electricity savings associated with major measures for IT devices and infrastructure equipment. Results suggest that 2008 demand was approximately 69 billion kilowatt-hours (1.8percent of 2008 total U.S. electricity sales) and that it may be technically feasible to reduce this demand by up to 80percent (to 13 billion kilowatt-hours) through aggressive pursuit of energy efficiency measures. Measure-level savings estimates are provided, which shed light on the relative importance of different measures at the national level. Measures applied to servers are found to have the greatest contribution to potential savings.