Hybrid photovoltaic-thermal (PV/T) collectors consist of a thermal collector in which a photovoltaic laminate is attached as a thermal absorber. In addition to their improved electrical efficiency, due to the decrease of the photovoltaic temperature, PV/T collectors have also thermal efficiency, which is the main issue of this research. The theoretical study of a photovoltaic-thermal system for domestic heating and cooling has led to the result that the system can cover a remarkable percentage of the domestic heating and cooling demands. Furthermore, the performance of the PV/T collector with respect to the variation of geographical region as well as to different total surface areas of the system was also investigated. According to the results in both cases, the variation of the above-mentioned parameters has notably affected the solar coverage percentage of the system. As a conclusion it could be claimed that the photovoltaic-thermal technology offers a remarkable solution to the problem of domestic heating and cooling, hence contributing to the reduction of energy consumption in houses. (C) 2005 Elsevier Ltd. All rights reserved.