In this study, the performance of a photovoltaic thermal system (PVT) is investigated in a numerical and experimental study. In the numerical part, the Taguchi method is applied to determine the optimum place and time of the PVT system. Moreover, the optimum parameters that are independent of the design of the PVT system are obtained to improve the performance of the system in a specific place and time. Using the specified optimum parameters, the performance of the system is investigated from the energy and exergy viewpoints, experimentally. In the experimental study, using the designed setup, the performance of a water based PVT system is compared with that of a conventional photovoltaic unit (PV). The experiments are performed on a selected day in August at the Ferdowsi University of Mashhad, Mashhad, Iran (Latitude: 36° and Longitude: 59°). The numerical results indicate that the most effective parameter on the performance of the PVT system is the coolant inlet temperature and its optimal value is 20 °C. Moreover, the total energy efficiency of the PVT system in the optimum working condition is 69.02 %. The experimental results reveal that the average output electrical energy of the PVT system is 6.27 % more than that of the PV unit. In addition, the average thermal energy and exergy efficiencies of the PVT system are 34.12 % and 0.72 %, respectively.