From the Silicon Valley www.mercurynews.com an article about the extent of UL testing.
At a nondescript office park on Zanker Road in San Jose, solar panels from manufacturers in Silicon Valley and around the world are going through tests of extreme endurance to make sure they are safe to put on your roof.
High-powered lamps expose panels to the intensity of "One Sun—" the universally accepted standard for simulating the most extreme amount of sun exposure a solar product might ever endure.
In the "wet room," solar panels are constantly soaked by spray nozzles, as if from a hard, pelting rain. In another part of the 20,000-square-foot facility, panels are subjected to several days of extreme shifts in temperature and humidity. Then there's the "hail impact" test: ice balls are shot at solar panels with a gun, to see how the panels would weather a serious hailstorm.
For the rest of the story http://www.mercurynews.com/top-stories/ci_14419424?nclick_check=1