The Energy Policy and Conservation Act of 1975 (EPCA), as amended, prescribes energy conservation standards for various consumer products and certain commercial and industrial equipment, including residential water heaters. EPCA also requires the U.S. Department of Energy (DOE) to determine whether more stringent, amended standards would be technologically feasible and economically justified, and would save a significant amount of energy. Accordingly, DOE established amended energy conservation standards for several classes of residential water heaters in an April 2010 final rule. DOE is amending its definitions pertaining to residential water heaters to clarify the applicability of energy conservation standards for residential water heaters that are utilized as a secondary back-up heat source in solar-thermal water heating systems. Specifically, DOE is creating a definition for "solar-assisted fossil fuel storage water heater" and "solar-assisted electric storage water heater" and clarifying that water heaters meeting these definitions are not subject to the amended energy conservation standards for residential water heaters established by the April 2010 final rule.