For decades, energy efficiency has been a critical strategy for meeting US electricity needs while saving consumers billions of dollars in avoided infrastructure costs. Just how big is energy efficiency today across the US electric power sector? This paper quantifies the energy savings and other benefits from a set of energy efficiency programs and policies. We examine the combined savings from appliance and equipment efficiency standards, utility-sector energy efficiency programs, and building energy codes. We find that since 1990, energy efficiency has become the third-largest electricity resource in the United States; without it, we would need the equivalent of 313 additional large power plants to meet the country’s energy needs. If savings from appliance standards, utility programs, and building codes reached their full potential, efficiency would become our nation’s largest electricity resource by 2030, providing a wealth of benefits.