In this paper the current-voltage characteristics of organic solar cells (OSC) is analyzed in terms of equivalent lumped parameter circuit at different level of insolation. In particular, starting from a circuital model based on a three-diode configuration, a set of formulas is proposed to describe the dependence of lumped circuital parameters on irradiance intensity and consequently the behaviour of organic solar cells from dark to high irradiance conditions. These expressions are achieved by using a trail and error approach applied to the fitting procedure of measured I-V curves at different irradiance levels. The achieved model with parameters dependent on irradiance level allows us to use a single circuit for the analysis of OSC under different operating conditions: this is the first step for the integration of OSC circuital model in photovoltaic system simulator, but also can furnish a novel point of view for the advance of the OSC applied research. Experimental measurements are used to setup the model and for its successive validation on two different kind of organic solar cells.