The photoelectric properties of Si nanowires (SiNWs) under interface confinement are investigated based on the atomic-bond-relaxation consideration and the detailed balance principle. An analytical model is developed to elucidate the interface confinement and power conversion efficiency (PCE). It is found that the band curvature and surface barrier height decrease with decreasing size. The interface recombination rate and PCE can be determined by the size, shell thickness and local interface conditions. Our theoretical results show evident improvement in the PCE of SiNWs under interface confinement compared to that of a bare nanowire, highlighting the feasibility of the epitaxial layer as a booster for highly efficient SiNW solar cells.