The current study presents a novel and straightforward approach for simulating photovoltaic/thermal (PV/T) systems using the commercial computational fluid dynamics (CFD) solver ANSYS-FLUENT. Instead of resolving the natural convection within the air gap between the PV and the glass cover, the effective thermal conductivity approach is implemented. Moreover, the solar radiation incident on the PV layer is directly included in the energy equation of the PV domain to evaluate the resultant power output and heat generation. The validity of these implications is proven by comparing predicted data with experimental data from the literature. Comparative results reveal a root-mean-square error of 7% and 2% for the PV temperature and the outlet air temperature, respectively. A comprehensive numerical analysis is also conducted for a PV/T system with and without finned surfaces. In the parametric study, the impacts of varying a number of design parameters, operating conditions, and weather data over a wide range are assessed. Results reveal that channel height and air velocity have the greatest impact on the overall efficiency and outlet air temperature of a PV/T system. An optimization study is also conducted using the response surface methodology to obtain optimal values of design parameters and operating conditions for this system. The highest overall efficiencies and outlet air temperatures are achieved in PV/T systems comprising narrow channel geometries, regardless of the operating conditions or weather data considered. Optimal conditions are achieved for a collector with a collector length of 1.5 m, a channel height of 1 cm, and an air velocity of 2.3 m/s. For the optimal design, overall efficiency and outlet temperature values are evaluated as 53.4% and 310.9 K, respectively. Sensitivity analyses also observe the impact of adding fins to the air channel, and it is concluded that the addition of fins improves the overall efficiency of the PV/T system by up to 19%. However, adding fins does not significantly affect the outlet air temperature; nor does it improve the overall efficiency of the PV/T system beyond a critical channel height.