In this paper, the shading ratio about to output the minimum critical power of a photovoltaic panel was determined by experiment and simulation. A 2.475[kW] poly-crystalline photovoltaic panel consisting of 11 modules was used in the experiment and its surface was covered with shading curtains, thus the amount of light incident to the modules were controlled. In order to compare with the experimental results, the experimental circuits were modelled with the CASPOC (power electronics electrical drives simulator) and module parameters were applied to it, by which the minimum critical power was calculated. As a result, the photovoltaic panel was about to generate the power when the 5th shading curtains were removed from the module surface, after then the output power linearly increased by removing the shading curtains. In addition, the CASPOC simulation results were similar to the experimental results in the abrupt decreasing pattern of power.