Pressure retarded osmosis (PRO) is one feasible technology that can be used to exploit the mixing energy from salt gradients which is commonly referred to as salinity gradient power or osmotic power. This paper focuses on the construction of iso-watt diagrams and their application for detailed evaluation of specific power in PRO on basis of membrane characteristics and site specific design parameters. Iso-watt diagrams are demonstrated to be a useful tool for investigation of the effect of variation in the membrane parameters, i.e. the water permeability, the salt permeability and the structure parameter, and how mutual changes in the characteristic parameters will impact the PRO performance. Thus, iso-watt diagrams can be considered an effective aid for goal-oriented membrane development. In addition, iso-watt diagrams can be considered to be useful also with respect to design related aspects, such as e.g. site selection. Further, membrane performance data are reported in the literature for various experimental conditions, which makes direct comparison difficult. The use of iso-watt diagrams enables an efficient way of presenting and comparing the efficiency of different membranes. This is demonstrated in the current paper for ten states of the art membranes. (C) 2015 Elsevier B.V. All rights reserved.