In order to improve the energy performance of the photovoltaic system, much effort has been spent on the research and development of hybrid PVT (photovoltaic-thermal) technology using water as the coolant. The fin performance of the thermal absorber is known to be one crucial factor in achieving a high overall energy yield of the collector. Accordingly, an aluminum-alloy flat-box type PVT collector was constructed, with its fin efficiency approaching unity. Its design is primarily for natural circulation and for domestic water heating purpose. Our test results showed that a high final hot water temperature in the collector system can be achieved after a one-day exposure. A numerical model of this photovoltaic-thermosyphon collector system was also developed and the model accuracy was verified by comparison with measured data. The energy performance of the collector system was then examined first, through reduced-temperature analysis, and second, as applying in the "hot summer and cold winter" climate zone of China. The numerical results are found very encouraging, and the equipment is capable of extending the PV application potential in the domestic sector. (c) 2005 Elsevier Ltd. All rights reserved.