This paper presents a method for optimizing pumping energy costs. It is designated for large, computer-controlled water-supply systems. The software tool that is based on this method is an integral part of an automatic control system, which is used for designing and updating the operational schedule in real-time. In order to optimize the energy cost, the method takes advantage of two basic conditions: (1) the cost of electrical energy varies throughout the day, and (2) the water system includes storage capacity, which enables a separation between the tight connection between source pumping time and consumer supplying time. The whole process consists of three main parts: (1) factoring the engineering problem of schedule design into the mathematical model of the linear programming (L.P.); (2) reaching the optimal solution by using the Simplex method; and (3) interpreting the mathematical result into the operation schedule.