Solar water heater (PATS) is one of the technology applications that serves to produce hot water to meet the needs of households and industries. In general, PATS installation is placed outdoors (outdoor) by utilizing the source of energy from the sun. This method has a weakness in investigating the influence of the magnitude of input energy because the intensity of the sun radiation fluctuates every time until it causes the absorption of the heat transfer fluid (HTF) and phase change material (PCM) on PATS Fluctuate. Therefore, it is necessary to do indoor research by providing the heat flux that can be varied so as to know the influence of heat Fux variation on thermal efficiency characteristics in collectors. Indoor research is do using a solar simulator that serves as a controllable heat source. Solar Simulator is placed on the top of the collector. Thermocouples are installed on the HTF and PCM parts. Temperature readings on thermocouples are used in temperature logger data and software on personal computer (PC) and laptop. Heat flux variations used are 800, 1000, and 1200 W/m2. The water heating process is carried out for 98 minutes using 2 LPM discharge. The water temperature Data in the tank is recorded. Testing is repeated by changing the heat flux variation. Data analysis is conducted to determine the thermal efficiency of collectors. The research results of each test show that there is a influence of the heat flux variation on the small thermal efficiency obtained by the collector. The higher the heat flux variation is given, the thermal efficiency of the collector is getting lower, likewise vice versa