In this paper, the environmental benefits or renewable energy systems are initially presented followed by a study of the thermal performance, economics and environmental protection offered by thermosiphon solar water heating systems. The system investigated is of the domestic size, suitable to satisfy most of the hot water needs of a family of four persons. The results presented in this paper show that considerable percentage of the hot water needs of the family are covered with solar energy. This is expressed as the solar contribution and its annual value is 79%. Additionally, the system investigated give positive and very promising financial characteristics with payback time of 2.7 years and life cycle savings of 2240 E with electricity backup and payback time of 4.5 years and life cycle savings of 1056(sic) with diesel backup. From the results it can also be shown that by using solar energy considerable amounts of greenhouse polluting gasses are avoided. The saving, compared to a conventional system, is about 70% for electricity or diesel backup. With respect to life cycle assessment of the systems, the energy spent for the manufacture and installation of the solar systems is recouped in about 13 months, whereas the payback time with respect to emissions produced from the embodied energy required for the manufacture and installation of the systems varies from a few months to 3.2 years according to the fuel and the particular pollutant considered. It can therefore be concluded that thermosiphon solar water hearting systems offer significant protection to the environment and should be employed whenever possible in order to achieve a sustainable future. (C) 2008 Elsevier Ltd. All rights reserved.