Energy efficiency programs are in wide use, whether administered by state governments, city governments, or utilities. Because energy efficiency is often a low-cost means for reducing power sector emissions, the U.S. Environmental Protection Agency (EPA) expects it will be broadly used to comply with the Clean Power Plan, which sets greenhouse gas standards for existing power plants. This fact sheet compares the treatment of energy efficiency under two types of Clean Power Plan compliance approaches: rate-based or mass-based emission standards.