In this paper we investigate the performance of an integrated solar photovoltaic and thermal (PV/T) liquid (water) collector using a computational simulation program. A detailed time-dependent thermal model was formulated to calculate and correlate the thermal parameters in a standard PV/T collector, including solar cell temperature, back surface temperature, and outlet water temperature. Based on the energy balance of each component of the system, an analytical expression for the temperature of the PV module and the water was derived. In addition, an analytical expression for the instantaneous energy efficiency of the PV/T collector was also derived in terms of thermal, design and climatic parameters. Built on previously published model, a new computer simulation program was developed and validated. The thermal simulation results obtained are more precise than those previously reported in the literature.