Photoelectrochemical (PEC) water splitting offers a promising way to produce hydrogen and harvest solar energy, however, its low efficiency has made it less economically attractive than other hydrogen production methods. Herein we present a numerical model of PEC cells considering quasi-fermi level splitting and interfacial kinetics to understand the charge transfer process and explore the approaches to increase the energy conversion efficiency. The non-linear change of photocurrent with light intensity under concentrated illumination is for the first time captured by a model. Based on the model, the operation regions of a PEC cell are mapped. Pathways to further promote the energy efficiency of PEC are proposed from the aspect of kinetics and thermodynamics. A new method that enables a precise evaluation of the theoretical boundaries of energy conversion efficiency of photocatalysts is developed taking into account the thermodynamics barrier.