Integrating a suitable thermal energy depository device is extremely necessary to enhance the performance of solar thermal energy systems by a reduction in the discrepancies between the supply and demand of electricity. The most effective and potential system for the conservation of solar thermal energy is the dependence on phase change materials (PCMs). Here, we focus on the preparation and characterization of nanoparticle (NP)-reinforced PCM-based composites. Fatty acids and paraffin wax PCM, reinforced with different NPs, are extensively discussed in this review. To describe the composition, morphology, and thermal characters of the arranged NPs and the composite PCM, scanning electron microscopy (SEM) and Fourier transmission infrared (FTIR) spectroscopy are employed. To define the chemical structure, chemical density, component size, and internal structure of composites and chemical characteristics, EDX/XPS is performed on a component PCM, respectively. Graphene incorporation improves the whole system's thermal conductivity. Both used in the TGA and DTG study were the thermal decomposition details and chemical absorption of the polymer PCM. This study, therefore, focuses on PCMs, their methods for the processing of the materials, materials, and thermal characterization during the last five years.