In this study, the cylindrical phase change energy storage tank linked to the water source heat pump system is investigated theoretically and experimentally. A simulation model defining the transient behavior of the phase change unit was used. In the solar energy storage tank, the phase change material (PCM) is inside cylindrical plastic tubes and heat transfer fluid (water) flows parallel to it. The heat transfer problem of the PCM was solved numerically using an enthalpy-based finite differences method and was validated against the experimental data. The experiments were performed for clear days during the winter season in Trabzon, Turkey. In the experimental study, the mean temperature of the water and the temperature distribution of the PCM in the tank were measured. The inlet and outlet water temperature of the tank and the inlet and outlet temperature of the solar collectors and the water-source heat pump are also measured. The experimentally obtained inlet and outlet water temperatures are also taken as inlet/outlet water temperatures in the simulation model. Thus the theoretical temperature and stored heat energy distribution in the energy storage tank have been determined by using a computer program developed previously. In this study, solar radiation on horizontal and tilted surfaces in Trabzon are also measured and calculated while the collector and storage efficiencies have been calculated. The theoretical results were found to be in good agreement with the experimental values.