This article presents a theoretical analysis of the efficiency and output power of electrokinetic energy conversion in fluidic nanochannels. Phenomenological equations are utilized to clarify thermodynamically the conversion principle between mechanical energy and electrical energy in electrokinetic flow. Such electrokinetic devices performance is gauged by a dimensionless “figure of merit” Z, a combination of phenomenological coefficients. We demonstrate that Z is dependent on four non-dimensional parameters: electrokinetic height, zeta potential, Levine number and Dukhin number.