Building energy modeling (BEM) is commonly used to estimate the energy usage of residential buildings. Uses for BEM include evaluating design options, calculating home energy ratings, demonstrating compliance with performance-based energy codes, and establishing whether designs meet voluntary program requirements, such as ENERGY STAR® qualified homes. BEM requires simulating subsystems within the building, including storage-type water heaters. Simulating the in-situ performance of residential storage water heaters requires values for key water heater parameters (including the overall heat loss coefficient (UA) and conversion efficiency (ηc) of the water heater) which can be derived from water heater rating data. The testing procedure and rating standard for residential water heaters have recently changed: the new rating standard provides a Uniform Energy Factor (UEF) rather than the Energy Factor (EF) used previously. This paper discusses how to derive the necessary model parameters from the ratings data produced from the latest test procedure.