Hybrid photovoltaic/thermal collectors (PVT) consist of common photovoltaic modules cooled by a suitable fluid, and convert solar radiation simultaneously into both thermal and electric energy. The heat transfer between PV cells and fluid allows to reduce the temperature of the PV cells; this improves the electrical efficiency, but also makes available low-grade heat for specific applications. As a consequence, PVT modules show a very interesting overall energy efficiency. In this paper, the Second Law analysis of a water-cooled PVT collector is presented, based on simulations. The study also discusses a crucial problem for the optimal exploitation of this technology: the electricity production from PV cells is favoured by low temperatures, whereas the usability of the thermal energy gets higher at high temperatures. The paper demonstrates that, for any operating condition, it is possible to calculate an optimum water inlet temperature that maximizes the total exergy generated by the system. The optimum temperature falls within the range commonly occurring in solar thermal systems, and can be achieved in practice through a simple control-command system. Finally, a thermoeconomic analysis is carried out to define the price of the thermal energy produced by the PVT collector, as a function of its exergy content. (C) 2014 Elsevier Ltd. All rights reserved.