The performance of nano-ionic liquids as working fluids for solar thermal energy storage systems is analyzed and compared to that of traditional water-based solar storage systems. Critical variables are evaluated via the experimental setup (e.g., heat capacity, collector instantaneous efficiency, and average tank temperature) to obtain the optimum nanoparticle concentration for maximum thermal performance. Results show a significant enhancement in the heat capacity of the ionic liquid 1-Butyl-3-methylimidazolium hexafluorophosphate [Bmim][PF6] with the addition of copper oxide nanoparticles (CuO). The best thermal performance is achieved with a 0.60% concentration of nanoparticles that provides a heat capacity increment of 34%. In addition, the instantaneous efficiency of the solar collector increased with the addition of nanoparticles to a peak efficiency of 74.17% at the 0.60% concentration. Moreover, the liquid phase temperature range of [Bmim][PF6] with CuO nanoparticles is significantly more expansive than that of water. It remains a liquid up to 200°C, compared to water at 100°C. This broader temperature range makes it highly suitable for high-temperature applications without the water phase change limitations. However, it's important to note that higher concentrations of nanoparticles can lead to aggregation and reduced thermal performance. In conclusion, our study underscores the potential of nano ionic liquids, with optimized nanoparticle concentrations, as a convincing alternative to conventional thermal storage media. They offer clear advantages in high-temperature applications and can significantly enhance the overall efficiency of solar collector systems.