A particle-swarm optimization method with nonlinear time-varying evolution (PSO-NTVE) is employed in determining the tilt angle of photovoltaic (PV) modules in Taiwan. The objective is to maximize the output electrical energy of the modules. In this study, seven Taiwanese cities were selected for analysis. First, the sun's position at any time and location was predicted by the mathematical procedure of Julian dating, and then the solar irradiation was obtained at each site under a clear sky. By combining the temperature effect, the PSO-NTVE method is adopted to calculate the optimal tilt angles for fixed south-facing PV modules. In this method, the parameters are determined by using matrix experiments with an orthogonal array, in which a minimal number of experiments have an effect that approximates the full factorial experiments. Statistical error analysis was performed to compare the results between the four PSO methods and experimental results. Hengchun city in which the minimum total error value of 6.12% the reasons for the weather more stability and less building shade. A comparison of the measurement results in electrical energy between the four PSO methods and the PV modules set a six tilt angles. Obviously four types of PSO methods simulation of electrical energy value from 231.12 kWh/m(2) for Taipei to 233.81 kWh/m(2) for Hengchun greater than the measurement values from 224.71 kWh/m(2) for Taichung to 228.47 kWh/m(2) for Hengchun by PV module which is due to instability caused by climate change. Finally, the results show that the annual optimal angle for the Taipei area is 18.16 degrees; for Taichung, 17.3 degrees; for Tainan, 16.15 degrees; for Kaosiung, 15.79 degrees; for Hengchung, 15.17 degrees; for Hualian, 17.16 degrees; and for Taitung, 15.94 degrees. It is evident that the authorized Industrial Technology Research Institute (ITRI) recommends that tilt angle of 23.5 degrees was not an appropriate use of Taiwan's seven cities. PV modules with the installation of the tilt angle should be adjusted in different locations. (C) 2010 Elsevier Ltd. All rights reserved.