Buffer tank is a type of surge tank which is predominantly used to regulate pressure fluctuation in a heat pump system. In this system, the photovoltaic-thermal collector plays a major role in retaining heat of water inside the tank, since an insulating material completely encloses the buffer tank. The main objective of the study is to utilize a buffer tank to reduce the energy consumption of an electric heater of a heat pump system. In accordance with industrial design standards, the design of the buffer tank is made and examined using Finite Element Analysis (FEA) by varying the pressure and mesh density. The varying pressure has been calculated theoretically at different portions of the tank, such as the pressure inside cylindrical shells, dished ends, and flange tubes. The internal pressure of 1.6 bar and the external pressure of 2 bar were obtained according to the tank specifications. The post-run findings of stress, strain, displacement, and factor of safety are analysed and are significant to determine the efficiency of the heat pump system. The observed result from the analysis shows that the maximum Von-Mises values are below the maximum permitted working stress. The tank is experiencing a reversible deflection of maximum 1.813 mm at the dish end of the buffer tank. It is significantly less and has no impact on the buffer tank, the heat pump system operates more efficiently.