Soft carbon and hard carbon materials are two of the most promising materials for sodium ion battery anodes. However, the relation between the Na storage mechanism and the microstructure of the carbon materials is not well understood. In this work, we extend our previous phenomenological model and develop an efficient mean field theory for the calculation of the OCV of carbon materials ranging from soft carbon materials to closed pore and open pore hard carbon materials. Our model utilizes a very simple model for the energetic sites within the microstructure of disordered carbon materials and aims for a unifying picture of the Na storage mechanism in carbon materials ranging from soft carbon to hard carbon. In addition, we also study the impact of the current density on the utilization of energetic sites in the microstructure of hard carbon materials.