Usually, stresses in module integrated solar cells are obtained by Finite Element (FE) calculations because these stresses can hardly be determined by measurements. Apart from the knowledge of the material properties, the FE boundary conditions have a distinct effect on the results of a FE solution for a standard mechanical pressure test. We used different simplified approximations to the complex real clamping of pv modules for simulating the deformation of the module and hence the

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... situation in the cells. Some deformation results could be compared to experimental findings from a standard mechanical load test. In this paper we show the bandwidth of results from variations of the assumptions which were made for the simulations. We found that particularly weak modules, i.e. thin frame and thin glass plate, show a strong dependency of cell stresses on the boundary conditions of the FE model. Hereby, the calculated stresses resulting from one assumption can easily deviate for more than 50% from the stresses using another assumption, even when the displacements are nearly equal and both lie closely to the measured values. Finally, we outline one possibility to directly in situ measure the cell strains what could help to resolve this uncertainty.