The response of the basal plane of graphite towards an external field has been investigated by DFT. Because of the low electronic density of states the field penetrates over a distance of several tenth of a nanometer and exhibits oscillations. These calculations have been combined with a simple model for the electrolyte solution, which is represented as an ensemble of hard spheres. The combined model explains quantitatively the capacity characteristics of graphite. The behavior of graphite is contrasted with that of typical metal surfaces like Ag(111), Au(111) and Pt(111).