Passive cooling systems have garnered much attention in recent years as a sustainable and cost-effective method of regulating indoor temperatures. Phase change materials (PCM) are a promising technology for such systems, as they can store and release large amounts of thermal energy during the phase transition process while maintaining the temperature in a very specific range. In this study, we investigated the performance of a passive cooling system using PCM modules and evaluated the effect of different variables on its cooling efficiency. Several tests were conducted, varying the ambient temperature, number of plates, and PCM type to determine the optimal conditions for the system. A PCM with a melting temperature of around 22 °C was used and was compared to ice. While ice showed a larger cooling effect, the advantage of the PCM emerged with elevated ambient temperatures. Compared to ice, and due to the smaller the temperature difference ΔT between the PCM melting temperature and the ambient temperature, the cooling effect of the PCM, lasted for a significantly longer time. Moreover, increasing the number of plates proved to elongate the cooling effect, due to increasing the overall thermal storage capacity of the system. Overall, our findings suggest that a passive cooling system using PCM technology can be an effective solution for regulating indoor temperatures. However, careful consideration must be given to the choice of PCM type and melting temperature, as well as the number of plates in the system, to optimize its performance.