This book attempts to develop a rigorous design tool for sizing and optimising thermosyphon solar water heaters based on operating and weather conditions of particular geographical region, and the material to be used for designing the system. TRNSYS simulation program is used to evaluate the thermal performance of the thermosyphon system.Two new components were added to the TRNSYS to calculate theoretically the characteristics performance of the collector and overall heat loss coefficient of the tank that would otherwise have to be determined experimentally. Genetic algorithm routine is used as an optimisation technique, the system cost is chosen to be the objective function. The design tool is used to find the optimum thermosyphon system design that best suits Libyan families according to weather condition of Tripoli.