The thermal performance of a multi-tank solar water heating systems under different climatic conditions, has been characterised by the solar fraction i.e. the fraction of needed energy, available from sun. An earlier model of Sodha et al. (1984) has been validated with experimental observations on different models of solar water heating systems. This model and the concept of solar fraction have been used to size the system for different climatic regions of country represented by Jodhpur (hot and dry), Srinagar (cold), Bombay (warm and humid), Mount Abu (cool), Delhi (composite) and Bangalore (moderate).