Promotion of renewable energies to substitute carbon-based energy has boosted the development of new membrane technologies based on Salinity Gradient Power (SGP) by Reverse Electrodialysis (RED). This paper is focused on providing a useful, feasible and robust tool for the design of this technology, able to predict the behaviour under different operational conditions, critical for RED performance. Therefore, open circuit voltage (OCV), internal resistance (R-i) and gross power (P) are evaluated. Furthermore, the model predictability has been validated with experimental results obtained working with three cases of study corresponding to seawater/ WWTP effluent, brines/brackish water and an intermediate concentration gradient scenario. Feed flow rate (Reynolds numbers from 2.7 to 13.6), and temperature (from 286 K to 297 K) have been also tested in a lab-scale set-up with 0.4 m(2) of membrane area; the maximum power achieved at 297 +/- 1 K was 0.66 W, 1.6 W and 0.3 W for the three cases respectively. The results highlight the strong influence of temperature and the dominance of the low compartment resistance on the process performance; thus, working with the highest possible SG does not always provide the best outcome, but a trade-off between SG and resistance of the dilute solution should be searched.