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Long-term stability of photovoltaic (PV) modules has been systematically investigated by indoor and outdoor measurements, based on about 2400 field test PV modules manufactured in 1990s. Investigation was focused on changes in the photovoltaic performance of the modules as well as the visual defects which may affect the reliability of the modules in the range of 20-30 years. All the measured module showed initial loss in Isc. Some of the modules showed reduced Isc due to cloudy discoloration or delamination at cell/EVA interface, increased series resistance Rs probably due to the degradation in electrode soldering, or breaking of the cover glass. Although the overall system performance showed no distinct change within the 10 years of field test, the present results suggest that the above degradation features can affect the reliability of 20-30 years.