Thermal storage using phase-change materials PCMs is an efficient technique for enhancing the efficiency of solar energy utilization. This paper presents an investigation of solar thermal energy storage systems using an encapsulated PCM working in a realistic environment (Basra, Iraq). Numerical investigating the thermal performance of the heating system by using CFD technics with finite volume method .Water is used as the heat-transfer fluid HTF as well as sensible heat material. While paraffin wax packed in small cylindrical galvanized-iron capsules is used as the latent heat-PCM material. The capsules are integrated into a commercially cylindrical storage tank for hot water. First, the advantage of using flat plate solar collector FPSC and PCM is demonstrated on an insulated water tank. Finally, a turbulent-transient-3D model was developed using the ANSYS FLUENT 2020 R1 and compared with the experimental results. It was found in this case that the water temperature reaches above 50°C. Remained hot at least 25°C higher than the ambient temperature throughout 16 hours due to the presence of PCM. Parametric analyzes are performed to study the effect of the HTF flow rate (3 and 15lit/min) and the amount of PCM (5 and 10kg) on the PCM melting process, the charging time, and thermal storage energy of the water tank. The results indicate that the best performance can be obtained with the highest flow rate and the highest amount of PCM 15lit/min and 10kg, respectively, according to the current study. The results proved good agreement between the measured and numerical work.