Energy harvesting from a concentration gradient by reverse electrodialysis in anodic alumina nanopores was numerically investigated. Power generation from the nanopores when they are placed between two reservoirs containing potassium chloride solutions with different concentrations was examined. The current–potential characteristics of the nanopores were calculated by solving the strongly coupled Poisson equation, the Nernst–Planck equation, and the Navier–Stokes equations. Alumina nanopore arrays were also investigated experimentally to obtain the proper values of the surface charge density for the numerical model. The effects of various engineering parameters, such as the pore length, pore radius, and concentration, on the power generation were investigated on the basis of the developed numerical model. Finally, it was shown that a power output density of 9.9 W/m2 can be achieved using the alumina nanopores. This indicates that the alumina nanopores have the potential to be used as ion-selective membranes for microbatteries and micro power generators.