There has been a considerable attention in recent years to the applications of soft nanochannels in electrokinetic energy harvesting because of their flexibility that allows easier control of the electrokinetic batteries. Despite the very tiny sizes of soft nanochannels, no attention has been given to the ionic size (steric) effects on electrokinetic energy conversion in these devices. Here, considering a hydrodynamically-developed flow in a slit geometry, a model is developed for streaming potential phenomenon in soft nanochannels by taking the steric effects into account. It is shown that considering the ions as point-like charges may lead to a significant overestimation of the fluid velocity, especially when the density of the fixed charged groups is high. The story is different for the streaming electric field as the inclusion of the steric effects leads to larger values of it, except for cases with a dense distribution of the fixed charges for which the opposite is true. The variations of the efficiency of the electrokinetic energy conversion with the ionic size is similar to those of the electric field and an enhanced efficiency of up to 40% is observed by the inclusion of the steric effects at moderate densities of the fixed charges.
