Electrodialysis desalination (ED) is an efficient process to desalinate brackish water and to minimize brine salinities to the level of seawater salinity. From the many models reported in the literature, some are simple but have a limited accuracy while others are complicated with a good accuracy. In this context, we explore the improvement of a fundamentally 'lumped' model by considering the voltage drop at the interfaces between membranes and solutions (which is the so-called Donnan potential) while conserving the model simplicity. The present model is developed by accounting for an electrical resistance of the Donnan potential in the total cell pair resistance. The results are presented in terms of total membrane area, flow path length, number of stacks and specific energy consumption. The results show that the improvement resulting from involving the Donnan potential was substantial for a high difference between the feed and product salinities as well as for low salinities (< 500 ppm) of the product water and low flow velocities (< 0.05 m/s). In general, the inclusion of Donnan potential improved the results from 4.4% up to 39.2% based on the studied cases.