What causes solar panel degradation and how is it measured?

PV degradation causes: light-induced degradation (LID - boron-oxygen defects in first hours, ~2%), potential-induced degradation (PID - ion migration in humid conditions), UV degradation (encapsulant yellowing), hot spots (from shading or cell mismatch), and mechanical stress (thermal cycling, wind, snow loading). Typical degradation: 0.5-0.7%/year for crystalline silicon, higher for thin-film. Measurement uses I-V curve testing, comparison to reference, and thermal imaging for hot spots. Warranties typically guarantee 80-85% output at 25 years. Quality manufacturing and proper installation minimize degradation.