A comprehensive model for the optical transmission is constructed and used to investigate the requirements for fitting accurately the experimental data of the optical transmittance at normal incidence of transparent conducting coatings of ZnO:Al deposited on glass substrates by ultrasonic spray pyrolysis. The model takes into account the Urbach tail absorption edge at the low wavelength region, the contribution of free carrier concentration to the weak absorption in the visible and near-infrared ranges, and the effect of scattering of light originated by the surface roughness of the films. The carrier concentration of the ZnO:Al films was measured experimentally by the Hall effect and dc-electrical conductivity measurements in the Van der Paw configuration. It is shown that all mentioned physical effects must be included in order to fit accurately the transmittance spectrum in the VIS-NIR spectral window. The full expression for the optical transmittance was used for choosing the optimal thickness of these films as transparent conductive contacts and the calculation of the figure of merit.