Photovoltaic-electrolysis water splitting (PV-EWS) is the most promising approach for high solar-to-hydrogen (STH) efficiency. The present PV-EWS systems achieve the highest STH performance by using a III−V triple-junction configuration, which, however, involves a complex and expensive manufacture process. Therefore, in this work, we demonstrate a III−V double junction device that can be used as an alternative to the III−V triple-junction device for high STH conversion efficiency of the PV-EWS systems. We estimate the STH performance via coupling world-recorded multi-junction photovoltaic (PV) and our experimented cell configurations with an EWS system. The results show that the III−V double junction, owing to the good trade-off between the efficiency loss and compensation, exhibits a higher STH efficiency than the III−V triple-junction. Furthermore, strategies for improving the efficiency of the III−V double junction device for low-cost PV-EWS system are discussed.