Current solar hot water (SHW) system modelling techniques, as applied in Australia, are inadequate for the demands of programs and policies that aim at widespread usage of SHW. Two major problems with current modelling techniques are the coarseness of modelling in a spatial context and the use of only two hot water load categories. These problems are described and suggestions are offered for improved modelling processes. Geographical information systems can be used in conjunction with existing SHW performance evaluation tools to produce models with much finer spatial resolution than those currently in use. It is recommended that categories used for evaluating SHW systems should be based upon household size. Methodologies for modelling SHW system performance that conform to these recommendations are described in this paper.