By extending the theoretical model previously formulated for the single unit problem, we fully analyze the performance of the reverse electrodialysis stack module aiming for electric power generation. In our prototype, each single unit is assembled in serial, where the mixing is allowed in connecting channels to exclude the ionic polarization. The numerical algorithm accompanying the orthogonal collocation on finite element method is applied with Legendre polynomial and Gaussian quadrature integrations to predict the ion concentration profile in each compartment, power, energy, and corresponding current densities. As the number of units increases from 1 to 8, the maximum power and the maximum current densities decrease from 9 to 1 mW/m2 and from 3.6 to 1.3 A/m2, respectively, but the maximum energy density increases from 1.5 to 4 mW h/m3. Pursuing the justification of the validity of our RED stack module, we determine the unique compartment thickness that makes each density maximum for the specified number of units. Power and current densities increase with increasing characteristic fluid velocity, while the energy density maintains constant (ca. 0.36 mWh/m3) prior to monotonic decreasing at Pe > 70.