By leveraging ordered arrays of silicon nanopillars (NPs) and developing a quantitative multiphysics model, this work reveals the complex interplay of surface charge, liquid properties, and geometrical parameters in these systems, including previously unexplored electrokinetic interactions. Notably, we find that ion-concentration-dependent surface charge, together with ion mobility, dictates multiple local maxima in open-circuit voltage, with optimal conditions deviating from conventional low-concentration expectations. Beyond electrokinetic parameters, we show that structural asymmetries generate an electrostatic potential, augmenting HV performance. Finally, for molar-level concentrations, we provide evidence of ion adsorption and charge inversion for several monovalent cations, enabling HV devices to operate even at such high concentrations. Overall, we can uniquely demonstrate a high power density output of 8 μW/cm2 at 0.1 M. Our work thus paves the way for the broader applicability of HV systems across salinity scales.