We are going to design an energy-saving system about the water supply of the building in Hong Kong. The system includes a centrifugal pump and an electricity generator. Nowadays, the design of the water supply for a building mainly is using the centrifugal pump to draw water from the ground to the water tower in rooftop. Then, the water will flow downward to different floors though the pipes. Therefore, we can obtain water at home. For our design, we are going to investigate a better model of centrifugal pump to give a higher efficiency, such as changing the number and direction of the fan blade, so it can use less power to draw the water. Also, we consider adding a turbine generator in the pipe. When the water flows through the turbine, it will push the turbine and the potential energy released by the downward water flow will finally be converted into electricity by the rotation of the turbine.