Electrical energy storage is one of the promising solutions of the irregular electricity demand issue. Pumped-storage plants account for most of the storage capacity worldwide. It is popular for high capacity, good responsiveness and high efficiency. It can smooth peak loads, therefore plays a vital role in balancing the grid loads and enhancing the grid availability. A pumped-storage station can be operated in generation, pumping or idle states. In order to obtain an optimal plant schedule, a stochastic optimization model is proposed and the sample average approximation method is applied. The proposed model is used to determine the number of units to be hourly scheduled in generation or pumping mode. The model aims to minimize a supply-demand disparity and shortfall function as a weighted score taking into account the hourly grid demand. Furthermore, the model considers the grid demand uncertainty by considering the main two different demand scenarios “Winter and Summer”, and many constraints which are limiting the operation of a pumped-storage station. The model is solved optimally with a case study derived based on the Ingula pumped-storage station in South Africa. Results show that our model proposes pumping-generating schedules that could eliminate the disparity between demand and supply for both summer and winter demand profiles. Furthermore, it could provide a robust schedule that could work for both demand scenarios.