The paper proposes a generalized energy storage (GES) model for battery energy storage systems (BESS), electric water heaters (EWH) and heating, ventilation, and air-conditioning (HVAC) systems. The analogies, including state of charge versus water temperature differential, are identified and explained, and models-in-the-loop (MIL) are introduced, which are compatible with the Energy Star and CTA-2045 general specifications and command types. Emphasis is placed on the proposed EWH model as it needfully fulfills a gap in present literature. The corresponding MIL has been implemented in the DER integration testbed, which was originally developed by EPRI, and satisfactorily validated against experimental results. A case study is included to illustrate that the daily “energy content” and “energy take” for BESS and EWH with mixing valve technology are comparable for typical residential ratings. The BESS, which requires more initial investment, has advantages in terms of flexibility for contributing to grid services, which are illustrated through a combined simulation and experimental study based on data collected from a field demonstration site with four smart homes
