system is also simulated with TRNSYS for comparison. The component TYPE 45a “flat plate collector with water tank” was used. In this TRNSYS component, the collector thermal performance is modeled based on the Hottel-Whiller Eq. [24]. The stratification in the storage tank is modeled using the type 38 algebraic tank component. The design parameters are given in Table 3. The annual electricity saving and payback period were found 1316 kWh/year and 8.36 years based on electric heater, or 306 m3 gas saving and 6.10 years payback period based on gas heater, respectively. Table 4 shows the breakdown of the investment costs of the three different thermosyphon solar water heating systems, which were estimated from the quoted prices of the local suppliers in Hong Kong.