Soiling or dust accumulation on the surface of solar panels blocks or reduces the sunlight which is converted into electric energy. It is considered one of the major causes of energy loss of solar panels, especially in desert environments. Here, we present an analytical model and Monte-Carlo simulation on how the dust accumulation affects the transmittance of sunlight and the photovoltaic performance. Using the Lambert-Beer-Bouguer law, it is shown that the transmittance of the light is reduced exponentially as a function of the area dust density. The mass extinction coefficients explain that finer dusts give rise to more reduction in transmittance for given area dust density. The Monte-Carlo simulation shows that the reduction in transmittance fluctuates more for larger particles. We calculate the dependence of the transmittance of light on two particle-size distributions: the normal and log normal distributions.