While planning a photovoltaic system, it is essential to avoid the shading of solar panels. In case it is not possible, the minimizing of its effects are important. The current thesis gives a general review on solar panels, examines their operation, and the effect of shading, with special regard for the currently most common, polycrystalline silicon solar panels. The effects of different shadings are identified by combining my own measurings, with the work of others. I measured the effect of hard, and soft shadows for both for single panel, and connected in series, and parallel. The main purpose of this thesis is to find the most disfavorable cases with the intent of evasion. The measurements took place by extremely low irradiation levels, and in some cases this caused qualitative difference in the measured diagrams compared to the diagrams measured by nominal irradiance levels. The thesis gives a view on the difficulties introduced by variant shades, and on the possible reductions of these.