Phase Change Materials (PCMs) can be utilised for thermal energy storage to close the gap between energy supply and demand, as the main drawback in solar thermal collectors is the absence of heat gain during night hours. The charging and discharging period of the PCM in the water storage needs to be examined and evaluated. One of the main difficulties to apply PCM in a solar thermal system is to identify its optimum size and observe its phase changing process. Therefore, numerical investigations were performed to identify the size of the PCM needed to accomplish a high performance system, which is done using the ANSYS FLUENT. In the present work the charging and discharging mechanism were covered and analysed numerically and experimentally. Moreover, several investigations were done to assess the PCMs characteristics and effect in the solar thermal application, which is by focusing on its selection, implementation technique and thermal conductivity. Numerical analyses were conducted via ANSYS Fluent and validated with experimental work. The most suitable PCM for the operating condition of a domestic solar thermal application was found to be paraffin wax. The optimum selection of the PCM was reached via CES Granta software and a constructed selection criterion. The implementation of the PCM in the solar thermal application was accomplished using the storage tank as a double pipe heat exchanger. Through the experimental analysis, the thermal conductivity enhancement is done by adding aluminium foam to the PCM, which indicates a significant effect on the performance of the charging and discharging period of the PCM. Numerical investigations were done on the material interface between the PCM and the water to observe its effect on the thermal conductivity. The material interface did not exhibit any significance on the thermal conductivity of the PCM. The optimum size of the PCM was attained using ANSYS fluent.