Through this paper we study losses that occur in a photovoltaic system by making simulations using the following software: Pspice and PVSYST, emphasizing the moment when the PV cells are shaded or covered, namely when the level of irradiation is not uniform throughout the module. In technical literature, the irradiation degree of solar panels is one of the determining factors for PV systems to function in optimal parameters. In this paper we modeled the effect of shading and covering of photovoltaic cells with PSpice software simulator.