The present study focuses on a parametric analysis on a composite-PCM structure aimed at enhancing the thermal storage capacity and inertia of a hot water tank. The inner surface of the tank comprises a conductive material (stainless steel) in direct contact with the stored hot water, while the outer surface is constructed using a combination of PCM and insulating material (Glass/PP composite). A constant temperature is considered on the conductive material surface, while the part exposed to the surroundings is assumed to be adiabatic. An organic PCM in solid-liquid state, specifically Rubitherm paraffin RT55, has been selected for this study. Previous research has identified PCM dimensions where the melting time approaches a constant value. The objective of this study is to investigate the impact of various temperature gradients on these parameters. Furthermore, the study has identified the presence of a maximum critical Fourier number and an optimal shape factor as the Stefan number tends towards infinity