This paper presents the effects of spectral irradiance distribution on the performance of polycrystalline (p-Si) and amorphous silicon (a-Si) photovoltaic (PV) modules under Thai climatic conditions. The annual outdoor solar spectrum measurements bouncing on the PV modules installed at Energy Park, School of Renewable Energy Technology (SERT), Naresuan University, Thailand revealed that the average photon energy (APE) of total incident solar irradiance was 50% higher than that at standard test conditions. The results show that the power output of a-Si PV module mainly depends on spectrum distribution. In contrast, the power output of p-Si is not sensitive to spectrum distribution. Because of the different spectrum response of p-Si and a-Si PV modules, the actual irradiance spectrum data of each location under real working conditions are necessary for optimizing PV design. (C) 2011 Elsevier Ltd. All rights reserved.