Solar water heater (SWH) is a technology that is widely used in daily life by the community, such as for bathing hot water on a household scale. Generally, SWH installations are outdoors, utilizing sunlight. However, this has a disadvantage because solar radiation is fluctuating so it requires other methods to get constant radiation. This study examines the capacity of collecting thermal energy in SWH involving PCM. HTF, PCM, capsules, and tanks are materials that have the function of collecting thermal energy in TES tanks. This research method was carried out using a solar simulator that has a function to supply a constant heat flux to the SWH in an indoor room. There are 25 thermocouples in the SWH tank, then the SWH is fully filled with water and the solar simulator is turned on. At that moment, PC (personal computer) and laptops with temperature data loggers and software were installed. Then, the solar simulator is set to produce heat flux variations of 800 W/m2, 1000 W/m2, and 1200 W/m2. Then record data on the temperature of the water in the tank. Testing is done repeatedly by varying the variation of heat flux. This data analysis was carried out in order to find out information about the effect of heat flux on the thermal energy collection capacity in the SWH-PCM tank. The results obtained from this study are the highest thermal energy accumulation is 7,734 MJ on 1200 W/m² heat flux variations. Then, the higher the heat flux, the greater the energy collected. Researchers found that HTF has an important role in thermal energy storage.