Battery systems are critical factors in the effective use of renewable energy systems because the self-production of electricity by renewables for self-consumption has become profitable for building applications. This study investigates the appropriate capacity of the battery energy storage system (BESS) installed in all-electric zero-energy powerhouses (AEZEPHs). The AEZEPH used for this study is a highly energy-efficient house. Its criteria indicate that all the electrical energy within the home is covered based on the generated electricity from onsite renewable energy systems, including that the annual net site energy use is almost equal to zero. The experiment for measured data of electricity consumed and generated in the buildings is conducted for a year (i.e., January through December 2014). Based on the measured data, patterns of the electricity consumed by the AEZEPH and generated by an onsite renewable energy system (i.e., photovoltaic (PV) system) and BESS's appropriate capacity are then analyzed and evaluated using the electric energy storage (EES) analysis tool, named Poly-sun. This study indicates that self-consumption can be increased up to 66% when the ESS system is installed and used during operating hours of the PV system. The amount of received electricity during the week tends to be reduced by about two times.