The solar water heating system is used in the small residential to the large scale commercial or even industrial heating applications in the low temperature range. Storage of solar energy is essential to ensure the reliability in the performance of the system which depend on it. Present work uses a PCM based thermal energy storage for the investigation. A rectangular domain filled with Polyethylene glycol 900- a PCM which has its melting point 34°C, is consider for numerical investigation. Exponential heating is given to left wall, bottom and top wall sequentially as three different cases. In all cases two adjacent walls is maintained to zero heat flux. The investigation has been executed by using COMSOL Multiphysics. The outcome of investigation results shows that in initial hours most of the heat transferred through the diffusion mode. Later the buoyancy effect dominates which leads to the natural convection. Heat transfer becomes stronger. Top and bottom wall heating shows more energy storage compared to the left wall heating. This happens because of the stronger buoyancy effect. The numerical work compared with the experimental result. This study is useful for the thermal energy storage system designers.