The paper proposes a novel approach to modelling time-varying non-linear loads in power systems, based on demand conditions. The model allows a pre-evaluation of harmonic disturbances under variable conditions using normal and uniform distribution to randomize the electrical values of the non-linear loads and the related harmonic spectra. A measurement campaign conducted on an industrial area, a deposit/maintenance and repair site utilized for trains in operation on the metro line B of Rome, has been taken into account to verify the model performance. The methodology allows estimating the voltage total harmonic distortion at the point of common coupling. Through a statistical analysis, the harmonic distortion distribution via common normality tests was evaluated. The procedure has been used also to a harmonic disturbance pre-evaluation due to a future introduction of electric vehicles charging stations in the area.