The paper gives an analysis of the performance of the photovoltaic modules with planar booster reflectors with variable length and tilts for Swedish conditions. The booster reflector for photovoltaic modules of crystalline silicon cells should be designed so that an even illumination over the module is obtained since the cells in the module are connected in series. However, if amorphous silicon or thin film modules are used, the module can be placed with the length of the cells extended north-south. In this geometry the cells which are connected in series in the module always obtain the same average irradiance, and reflectors of a shorter length can be used compared to a design where the whole module has to be evenly illuminated. A specular booster mirror with reflectance = 0.8 in front of an east-west aligned PV module with one third of the reflector area increases the annual irradiation by about 25% with allowance of uneven illumination on the module plane and no requirement for tracking. The additional cost for the reflectors is of the order of 10 % of the cost of the modules, which means that installation of a booster mirror is cost effective compared to increasing the module size.