Solar water heating collectors are considered one of the most important options for providing thermal energy to buildings due to their use of renewable energy, reduced fossil fuel consumption, and low operating costs. In this study, the performance of a solar water heating system for a residential building in the cities of Tehran and Kashan was evaluated dynamically and over a one-year period using the TRNSYS software. The results showed that solar radiation in both cities has a specific seasonal behavior; such that the highest amount of radiation is observed in the warm season and the lowest amount is observed in the winter. The energy analysis results showed that the useful energy produced by the collector was directly related to the intensity of radiation and reached more than 5000 kJ/h during peak radiation periods. Although heat losses increased with increasing collector operating temperature, the useful energy produced prevailed over losses during most hours of the year. Also, the need for auxiliary energy was almost zero in the summer and increased in the winter. The thermal behavior of the storage tank showed that the water temperature was in a stable range during most hours. The annual efficiency of the collector was 57.37% in Kashan and 55.08% in Tehran, indicating the optimal performance of the system in both climates. Overall, the results indicate that the system is able to adequately supply the thermal load of the building and that areas with higher radiation, such as Kashan, provide more optimal conditions for the operation of solar thermal systems.