The accumulation of dust on the surface of a photovoltaic module decreases the radiation reaching the solar cell and produces losses in the generated power. Dust not only reduces the incoming radiation on the solar cell but also changes the dependence on the angle of incidence of such radiation. This work presents the results of a study carried out at the University of Malaga to quantify radiation losses caused by soiling on the surface of photovoltaic modules. Our results show that the mean of the daily irradiation losses in a year caused by dust deposited on the surface of a photovoltaic module is around 4%. After long periods without rain, daily irradiation losses can be higher than 20%. In addition, the irradiance losses are not constant throughout the day, and they are strongly dependent on the angle of incidence and the ratio between diffuse and direct radiations. The irradiance losses as a function of solar time are symmetric with respect to noon, where they reach the minimum value. We also propose a simple theoretical model that describes the qualitative behaviour of the irradiance losses during the day. This model takes into account the percentage of dirty surface and the diffuse/direct irradiance ratio. Copyright © 2012 John Wiley & Sons, Ltd.