The solar modules account for about half the value of the whole system. Therefore, if the modules have a serial defect, a large part of the investment may be at the risk of being lost. Internal module damage is not normally visible to the naked eye. An examination with electroluminescence reliably reveals to the experienced technician different types of defect in a module and their effects on the performance of the system. We perform electroluminescence checks with a very high-resolution system and by trained and experienced personnel.