To study degradation of PV systems, we measured current-voltage (I-V) curves from 250 modules that have been deployed in grid-tied systems for 2 to 12 years in Arizona. We also documented visual signs of weathering. The I-V curves exhibit a variety of imperfections and considerable dispersion of P-max values. Although many of these modules appear weathered, we found few significant correlations between visible signs of weathering and lower P-max values, and the most degraded modules have no correlation between visual defects and performance. This indicates that important forms of degradation can remain hidden to the eye.