Solar water heating systems are widely used in Brazil for domestic purposes in single-family households. The exploitation of the potential energy of the water from the upper tank and the thermosyphon phenomena for hot water circulation constitutes the absolute majority of the residential solar water heating systems in the country. But, these water heating systems are usually sized according to tables provided by the manufacturers, which show the number of plates required based on the size of the family and the number of hot water outlets. This sizing is based much more on intuition rather than on scientific data. For that reason, this work has developed an optimization model for water heating systems design parameters, using a numerical simulation routine, in a long-term transient regime. The optimized design gives the slope and area of the flat plate collector, which results in the minimum cost over the equipment life cycle. The computing procedure was executed considering specific characteristics of the project. A thermosyphon solar water heating system with flat-plate collector for Sao Paulo's climate was simulated. The practice of Brazilian designers and manufacturers is to recommend the maximization of the energetic gain for the winter. This paper has analyzed in economic terms if it is more attractive to increase the gain of solar energy in the winter period, with the consequence of reduction of the solar energy gain along the year, or to adopt the adequate slope, which improves the yearly solar energy gain. (c) 2005 Elsevier Ltd. All rights reserved.