Solar panel efficiency decreases over time and this is referred to as degradation. The degradation rate depends largely on module technology, weather and quality of materials, however the industry standard rate is around 0.5%/year. Panels in moderate climates such as the northern United States had degradation rates as low as 0.2% per year. But the rate could be as high as 1% in more extreme climates.
NREL Degradation Study: