PATS research is generally conducted outdoors using solar energy. But outdoor research has shortcomings, because the supply of solar energy is volatile. This has an effect on the fluctuating thermal behavior results of HTF and PCM. PATS research needs to be done on active systems using HTF and PCM which are carried out indoors. To get the thermal behavior characteristics of the heating process in HTF and PCM in the PATS tank. This study uses water as a Heat Transfer Fluid (HTF) and PCM in the form of paraffin wax as a heat storage medium. This research was conducted during the charging process using 3 variations of heat flux namely 800, 1000 and 1200 W/m2 with a discharge of 2 LPM. The charging process uses a solar simulator to simulate solar radiation. The collector heats the water that is pushed by the pump into the tank. The installed thermocouple will be recorded temperature every 1 second in the data logger. The results showed the temperature rise of each heat flux was not much different. The fastest increase in HTF and PCM temperatures occurs in the variation of 1200 W/m2 heat flux during the 98 minute charging process. This shows that the greater the heat flux, the higher the temperature of HTF and PCM will be faster. This research shows the thermal behavior is quite good, because during the charging process the curve has increased which is quite stable