This study presents a theoretical analysis of the dynamic behavior and thermal performance of a solar water heater with an internal exchanger using a thermosiphon system. The heat exchanger, made of a helical coil, is placed in the horizontally cylindrical storage tank so that the solar hot fluid crosses a significant mass of stored water. In order to analyze the dynamic natural circulation of solar water heaters by thermosiphon, a new TRNSYS component was developed using W-language, based on a detailed mathematical model. As a result of simulation, the solar heat yield and the average solar fraction were 2,001 kWh and 75.1%, respectively. The lowest solar fraction in January was 50.5%, and then gradually increased, reaching almost 100% from May to September. Here, when the system overheating lower limit temperature is set to 90℃, it was exposed to the overheating period for about 153 hours, which is 6.5% of the annual operating time (about 2,339 hours). Secondly, the instantaneous maximum efficiency of the collector was 42.3%, and the annual average system efficiency considering the heat loss of the solar storage tank was 36.2%.