Reduced dependence on fossil fuels is important for environmental and economic reasons, and this is why energy storage technologies are so vitally important. “Phase change materials” (PCM) are ideal choices due to their high latent heat storage capacity. When building a system to store energy, it is crucial to think about factors including the materials used, the operating conditions, and the geometrical arrangements. The primary goal of this research is to evaluate the melting time and heat transmission processes for the rectangular enclosures containing the same volume of polyethylene glycol 1500 as the PCM and completely isothermal walls via the use of a computer simulation. The fins attached to the enclosure are meant to decrease melting time and increase heat transmission to the PCM. Some industrial applications (heating-cooling systems & air conditioning, wall panels, the automotive industry, & the textile industry, to name a few) require full contact of the enclosure’s walls with the surrounding fluids like air, oil, and so on. The findings showed that natural convection was more influential in melting PCM in rectangular enclosures with 14 fins because of the greater contact area between the heated wall and fins’ wall. Case 4 had a greater amount of liquid fraction at any given moment, as shown by the data.