25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 3745-3752, Micro cracks in wafer based silicon solar cell modules are nowadays identified by a human observer with the electroluminescence (EL) method. However, the essential question of how the micro cracks affect the PV module performance has yet to be answered. We experimentally analyze the direct impact of micro cracks on the module power and the consequences after artificial aging. We show that the immediate effect of micro cracks on the module power is small, whereas the presence of micro cracks is potentially crucial for the performance of the module after artificial ageing. This confirms the necessity to develop the means of quantifying the risk of power loss in PV modules with cracked solar cells in their lifetime, in order to enable manufacturers to discard defective modules with high risk of failure while keeping modules with uncritical micro cracks. As a first step towards a risk estimation we develop an upper bound for the potential power loss of PV modules due to micro cracks in the solar cells. This is done by simulating the impact of inactive solar cell fragments on the power of a common PV module type and PV array. We show that the largest inactive cell area of a double string protected by a bypass diode is most relevant for the power loss of the PV module. A solar cell with micro cracks, which separate a part of less than 8% of the cell area, results in no power loss in a PV module or a PV module array for all practical cases. In between approximately 12% to 50% of inactive area of a single cell in the PV module the power loss increases nearly linearly from zero to the power of one double string.