The performance of six photovoltaic (PV) modules composed of polycrystalline silicon (pc-Si), amorphous silicon (a-Si), and hydrogenated amorphous silicon/crystalline silicon (a-Si:H/c-Si) modules was investigated at eight locations in Japan from August 2007 to December 2008. In addition, solar irradiance, solar spectrum, and module temperature were simultaneously measured in these round-robin measurements. In this study, we evaluate quantitatively the effects of module temperature and solar spectrum on the performance of the PV modules as thermal factor (TF) and spectral factor (SF), respectively. Furthermore, we investigate the variation in module performance, which is converted into module performance under standard test conditions (STC) using the TF and SF. In the case of the pc-Si modules, the variations in performance ratio under STC (PRSTC) for these modules range from 0.056 to 0.074 through the round-robin measurements. The TF indicates that the contribution of module temperature to the variation in performance is large, between about 15 and 20%. However, the SF suggests that the contribution of solar spectrum is quite small, less than 3%. In the case of the a-Si modules, the contribution of module temperature is about 8%. The performance is largely influenced by solar spectrum, more than 12% at its maximum. Consequently, the variations in the corrected PRSTC of the a-Si modules are between 0.117 and 0.141. These large variations may result from the effects of thermal annealing and light soaking. The variation in PRSTC of the a-Si:H/c-Si module is similar to that of the pc-Si modules. Copyright © 2010 John Wiley & Sons, Ltd.