H.P. Garg Centre of Energy Studies Indian Institute of Technology Hauz Khas, New Delhi 110 016 India Heating of water using solar energy is not new and by using a little science and technology in it, the solar energy can be utilized more effectively and economically for heating the water both for domestic and industrial applications. Solar Water Heaters are popular for the last three decades in countries like USA, Australia, Israel, Japan, India. This is the only solar energy application which is commercially, technic­ ally and economically viable and has been studied for more than 30 years in many countries. Technical advances in solar water heating have been very rapid in the last 30 years. These are becoming popular not only for domestic use but for large establishments like hostels, hotels, hospitals, industries such as Textile, Paper and Food Processing and even in heating of swimming pools in winter. In few instances the cost of solar water heating systems may be higher than those operated by electricity, gas or other fuel but over a period of time this is more than recovered by the savings in the cost of operations and maintenance.