In order to improve the transient performance of photovoltaic/thermal (PV/T) collectors, this paper first developed and validated an improved computational thermal model to investigate the effects of the major control parameters on the thermal performance of PV/T collectors, including inlet water temperature and inlet water flowrate. Second, a computational electrical model of PV/T system, coupled with the thermal model, was also introduced to elaborate the relationship of voltage, current, and power of a PV module (MSX60 polycrystalline solar cell) used in an experiment in the literature. The thermal and electrical models were solved simultaneously to predict the transient energy conversion efficiencies of the PV/T system. Simulation results were found to agree with the experimental data very well. The effects of the time-steps from 1 h to 1 min, which is close to real-time, were reported at various conditions. It was found that both thermal and electrical efficiencies are fluctuating with time. There is an optimal water flow rate at which the efficiencies are at the maximum. Suggestions to improve the efficiency of PV/T system were discussed.