Pressure retarded osmosis (PRO) produces energy using the salinity gradient between two solutions (draw solution (DS) and feed solution (FS)). Net energy production (NEP) of PRO was analyzed using a module-scale model developed in this work. The NEP analysis determines net energy from PRO by the difference between energy production by turbine and energy consumption by DS, FS, and booster pumps. Especially, the effects of system capacity and membrane fouling on NEP are investigated using a module scale modeling approach for the first time. The maximum net specific energy (NSE) per PRO system capacity (sum of DS and FS flow rates) is close to 0.1 kWh/m(3) without pretreatments. The maximum NSE decreases at smaller system capacities, and it becomes around 0.03 kWh/m(3) from a PRO system with 520 m(3)/d as capacity. NSE from seawater decreases in the presence of membrane fouling, but it remains positive under the severe fouling condition where water flux decreases by 32% if the system capacity is large enough to have efficient pumps and turbines. (C) 2018 Elsevier Ltd. All rights reserved.