The effects of aging and environmental factors on the performance of cadmium telluride (CdTe) and copper indium gallium selenide (CIS) thin-film photovoltaic (PV) modules have been analyzed. Recent developments in thin-film technology have resulted in reduced manufacturing costs and increased cell efficiency, making thin-film PV modules desirable for future solar-energy-generation systems. Such PV modules are currently used for various power system applications; however, their performance during the operational lifetime is greatly affected by aging and module failures caused by environmental factors. In this study, x-ray diffraction and infrared radiation imaging were used to characterize the imperfect surfaces of CdTe and CIS samples taken from a real application under various aging and environmental conditions. The performance of the modules was also evaluated by measuring the current, voltage, power, and efficiency over 3.5 years. During this period, we mainly detected corrosion, delamination, micro-cracks, and hot-spot failures. At the end of the study, the module degradation factors (MDFs) were calculated based on the measured results. The micro-crack and delamination failures resulted in MDFs of 14.49% for the CdTe modules and 21.24% for the CIS modules. With these MDFs, the installed system inevitably experienced substantial power loss in generation: 21.7% for the CdTe modules and 31.5% for the CIS modules. Consequently, our findings indicate that the performance of a PV system is susceptible to aging, meteorological conditions, and the PV technology; these factors should thus be considered before implementing PV system designs to achieve the most efficient and cost-effective power generation.