A site-saturation model for the variation in optical efficiency of tungsten oxide-based intercalation devices is presented. The model applies to situations in which a dominant fraction of the electrons associated with the intercalating species are appreciably localized. It is predicted that the optical efficiency diminishes with increasing intercalation level, reaching zero when half the sites are occupied. The model is quantitative and predictive. The model is compared to several sets of data which it matches well. The only parameters of the model are film thickness, maximum obtainable optical density, and relative density of the film. The range of suitability of the model and the interpretation of its comparison with experiment are discussed.