The chemical potential difference of two liquids with dissimilar salinities can produce electrical energy using salinity gradient power (SGP). The pressure-retarded osmosis (PRO) concept can be used to generate SGP across a semi-permeable membrane. This work describes a mathematical model for evaluating PRO processes at both the bench scale and the commercial scale. The effect of concentration polarization is considered, including concentrative external polarization. Bench scale simulation results show agreement with published experimental data. At the commercial scale axial variations in flow rates and concentrations are considered. The model is used to evaluate performance of membranes that have previously only been considered at the bench scale. Commercial-scale simulation results show power densities of up to 5.6 W/m2.