Thermal energy storage is one of the methods used in reducing energy consumption and heat recovery from thermal systems. Some of the major applications of heat storage systems are in buildings, solar energy technologies, and waste heat recovery from high-temperature applications. Thermal energy storage could be in the form of sensible heat, latent heat, or chemical heat arising from bond breaking and formation in chemical reactions. Among all these forms of solar energy storage aforementioned, latent heat thermal energy storage is greatly explored because it is generally safe to use and its ability to store a large quantity of heat in a relatively small volume. Phase change materials (PCMs) are widely used in latent heat thermal energy storage systems. Some of the setbacks encountered with the use of PCMs are its low-thermal conductivity and subcooling characteristics. Nanoparticles have been found very useful in improving the thermal conductivity of PCMs, thereby improving its general performance. The nano-PCM considered in this work is (graphene + octadecanol). The top surface of the nano-PCM enclosure was exposed to ambient solar radiation. The solar radiation data for Johannesburg (26.2041° S, 28.0473° E) was used for the simulation. A two-dimensional heat transfer equation was solved numerically using initial, boundary, and phase transition conditions. The effect of volume fractions of nanoparticles on the melt fractions was investigated. From the result obtained, the PCM with 4% loading of nanoparticles yielded the best result.