A model for pressure-retarded osmotic (PRO) power systems is described. The model considers several non-ideal phenomena including internal and external concentration polarization, local variation due to mass transfer, pressure losses along membrane surfaces and other losses throughout the system. This provides an overview of many of the major dynamics that must be considered in PRO power modeling. The model is validated by comparison to experimental data available in the literature. The model is used to investigate the effect of feed and draw flow rates, and of hydraulic pressure difference on PRO system performance. These parameters can be controlled by the system operator and can be set so as to minimize competing non-ideal effects. Improvements in net power of up to 7× are observed when best operating parameters are used as opposed to other values used in the literature.