A detailed physical model of a hybrid photovoltaic/thermal system is proposed, and algorithms for making quantitative predictions regarding the performance of the system are presented. The motivation for the present work is that solar cells act as good heat collectors and are fairly good selective absorbers. Additionally, most solar cells increase their efficiency when heat is drawn from the cells. The model is based on an analysis of energy transfers due to conduction, convection and radiation and predicts the amount of heat that can be drawn from the system as well as the (temperature-dependent) power output. Special emphasis is laid on the dependence of the fin width to tube diameter ratio. We attribute values to the model parameters, and show that hybrid devices are interesting concerning system efficiency as is also confirmed by previous experiments. Possible applications of such systems are also proposed.