There are many factors. For example surface damage to the panels. This happens through exposure to wind and weather, and the more obvious physical damage such as trees, dust, hail, and so on. If there are scratches on the surface, you may in a worst case scenario have water seep through the surface, and thus short-circuit the solar panels. So, you need to constantly make sure that the surface of the panels are clean, because pollen, dust, and so on lowers the efficiency of the cells.Has anyone in the industry done accelerated exposure testing? If so, what fails?
It has also to do with the chemistry behind it all. They are designed to react to photons hitting its surface of the cell. This process affects its longevity. If one or more cells are in the shade for far too long can create so called hotspots, and that accelerates the degradation as well.
Here's Fraunhofer on this issue of longevity: http://www.fraunhofer.de/en/press/resea ... les-7.html
And here's more on cell degradation: http://pveducation.org/pvcdrom/modules/ ... lure-modes