Dust deposition on solar photovoltaic (PV) systems mounted on windward roofs of an isolated building was investigated by CFD simulation. The SST k-ω turbulence model with UDF inlet profiles and DPM model were adopted to simulate the wind flow fields and dust deposition behaviours, respectively. The wind flow velocity profiles around the building were validated very well with the experimental results of the literature. The influences of dust sizes on dust deposition rates on PV panels were investigated in detail. The dust deposition rates first increased and then decreased with the increase of dust size. The maximum deposition rate was about 0.28% for 10 μm dust while the minimum deposition rate was about 0.13% for 50 μm dust. Moreover, a simple empirical model was developed to estimate the PV efficiency reduction ratios with exposure time based on the present CFD results and the experimental data of the literature.