Problems with the inverters/charge page | 57 controllers accounted for 1099 sets (20.9%) followed by 657 problems with batteries (12.5%) and 363 problems with wind turbines (6.9%). On-site investigations suggested that the bearings of wind turbines were made out of unsuitable copper material. The main causes of the exceptionally high proportion of malfunctioning charge controllers/inverters were poor technical design and the utilisation of unsuitable material during assembly.