A computer simulation program is used to predict the performance of a solar thermosyphon domestic water heating system. The model has a standard configuration with 4 m(2) of solar 250 l geyser which may be either vertical or horizontal. Typical meteriological year 15 min solar insolation data for the Pretoria area and corresponding ambient and cold water temperatures are used for the simulation input. A daily hot water requirement of 250 l at 55 degrees C is drawn off according to a specified pattern. Electrical elements ranging from 0 to 6 kW are thermostatically controlled to set the temperature of the top 1/3 of the geyser. It is found that a solar-only system will not provide a reliable source of hot water, but that with a 1 kW element a daily availability of at least 96% of the hot water energy requirement will be provided. Using a larger element will not significantly improve this availability, but will reduce the solar efficiency of the system by up to 3% and increase electrical consumption, In terms of a heating coefficient of performance, a vertical geyser would provide an annual average COP of 3.9 with a 1 kW element, reducing to 3.0 with a 6 kW element: a horizontal geyser is less effective with, a COP of 2.1 for the 1 kW element. If solar water hearing is to play a role, in a national energy-management strategy, the electrical maximum demand imposed on the generating system is of importance. It is found for the specified hot-water draw-off schedule that with a 1 kW element, a maximum half hour demand of 1 kW occurs, predominantly in the 07h00-10h00 and 20h00 periods, while a demand exceeding 1.5 kW is likely if the normal 3 kW element is employed.