Degradation and recovery tests have been conducted on recent single-junction a-Si solar cells under various light intensities and temperature conditions to predict long-term stability. Saturation phenomena of the degradation of efficiency have been shown experimentally. The degradation characteristics are expressed as the second extreme distribution function of the largest value with saturation introduced. The dependence of saturation on the cell temperature and the light intensity are investigated. The worst efficiency of seasonal changes over a long term can be predicted from the saturation characteristics.