A power plant in the south of Austria built with multi-crystalline PV modules was investigated after a hail storm. Standard measurement techniques like thermography, electroluminescence and ultraviolet fluorescence were used. It was found out that glass broken modules were not only responsible for a loss in power and yield. The remaining good-looking modules had invisible to the eyes cell damages causing the loss. Because of done measurements and documentation of impressive findings, the insurance decided to change the most affected bad working modules subsequently.