Solar photovoltaic as a safe and clean technology has been used to solve the problems posed by environmental factors and the energy crisis. However, it is more difficult to measure and calculate solar radiation and its power generation throughout the year without interruption. This study proposes a method to accurately assess the power generation of photovoltaic modules in complex weather conditions. Firstly, the maximum power point under different radiations is analyzed using a solar simulator, and a prediction model for the maximum output power of photovoltaic modules is established. Secondly, based on the output power model, the power generation efficiency calculation equation (dimensionless) of the photovoltaic module is derived, thus the relative power generation efficiency model is established. Then, using the measured data of annual solar radiation, an annual average efficiency model was proposed based on the radiation frequency distribution method (RSD method). Finally, the difference in annual power generation between photovoltaic modules in winter and summer was evaluated. The results show that the power generation in Tianjin is 87.61 kWh and 26.62 kWh in summer and winter, respectively, and the photovoltaic power generation in summer can reach three times of that in winter. The proposed model of annual average power generation of solar photovoltaic systems can accurately assess the annual power generation and power generation efficiency of photovoltaic panels, thus promoting the efficient utilization of solar energy resources.