The paper deals with an optimal technical solution to realize autonomous water pumping systems combining a PV electric energy source with battery chemical storage to supply a centrifugal electrical pump. The goal is to accomplish an optimal trade-off between the rated peak power of the PV generator and the power capability of the battery system in order to let the battery assisted solar pumping system operate at high efficiency while maximizing water storage. A novel method to evaluate the overall amount of delivered water has been developed. Results obtained by simulations confirm the effectiveness of the proposed method.