The performance of thermosyphon flat-plate solar water heater has been analyzed with the TRNSYS simulation program. In accordance with the national testing standard methods, the efficiency of the system was tested. The effect of installation height between the bottom of the tank and the outlet of the collector on efficiency has been investigated both theoreticaly and experimentally. The data show the theoretical results are highly consistent with the experimental ones. Results indicate that the optimum value of installation height is 0.74m with the maximum efficiency of 67.5% for the system with a total area of 1.5m² and storage water tank capacity of 120L. Increasing or decreasing the installation height within a range of 0.3m has no significant effect on thermal efficiency.