Degradation of Si-based photovoltaic (PV) modules after longtime outdoor exposure is quantitatively investigated using multiple regression analysis. Performance ratios (PR) of the PV modules are primarily influenced not only by irradiation intensity on tilted surface (IrrT), average photon energy (APE), and module temperature (Tmod) but also by angle of incidence (AOI). It is demonstrated that multiple regression analysis allows us to independently and quantitatively identify how each IrrT, APE, Tmod, or AOI (environmental factors) relates to the PR (outcome), thus leading to the understanding of impacts of environmental factors on physical properties of the Si-based PV modules. After longtime outdoor exposure, the influences of IrrT, APE, and AOI on the PR of multi-crystalline (mc-Si) and single-crystalline (c-Si) modules are severely increased, resulting from the decay of interconnection and discoloration of ethylene vinyl acetate films. The dependences of environmental factors on the PR of hydrogenated amorphous (a-Si) modules do not change as much as those on PR of me-Si and c-Si modules do because of differences in the module structures. (C) 2016 Elsevier Ltd. All rights reserved.