The paper is focused on the optimization and developing of design methodologies of storage water heaters for apartment buildings determined by experimental measurements in the different apartment buildings in Slovakia. The hourly maximum consumption of hot water in different apartment houses, the average hourly maximum of domestic hot water requirements determined for one person and energy balance of the domestic hot water system were found out by experimental measurements. Design methodology is composed of two steps. The first step is volume design of storage water heater and the second step is its power requirement design. This design methodology seems to be more accurate than the methodology of domestic hot water storage design in the Slovak Technical Standard currently in force. The proposed methodology better reflects the current demands of domestic hot water preparation in apartment buildings in Slovakia.