To analyze the effects of the module temperature and irradiance on photovoltaic (PV) modules outdoors, contour graphs were made for the performance ratio (PR) of single-crystalline Si (sc-Si) and amorphous Si (a-Si) PV modules. The result shows that the PR of the sc-Si PV module decreases with increase in module temperature. In contrast, the PR of the a-Si PV module shows unique temperature dependence influenced by temperature history. Dependence of environmental factors of the PR of the a-Si PV module was analyzed in almost the same temperature seasons, i.e., Spring and Fall. The PR in Fall was higher than that in Spring. The result shows that the recovered performance in summer may continue for Fall. The contour graph of remainder of PR in Fall and Spring was made. The environmental condition that the PR in Fall was higher was clarified. (C) 2008 Elsevier B.V. All rights reserved.