Energy efficiency of water supply systems in high-rise residential buildings becomes a concern for sustainable development nowadays. This paper presents an energy efficiency evaluation measure for water supply system designs that utilizes the potential energy of the water demands at end-users in buildings divided by the pumping energy required. A mathematical model is formulated for optimizing pumping energy through the arrangement of water tanks in a building. Measurement data of some high-rise residential buildings in Hong Kong are employed to demonstrate the model applicability. The results show the energy efficiency of many existing high-rise water supply systems is about 0.24 and can be improved to 0.26-0.30 via water storage tank relocations, corresponding to an annual electricity saving potential of 192 TJ, a 0.1% of the total annual electricity consumption in Hong Kong. The proposed measure of energy efficiency can be a parameter of designing sustainable water supply systems for buildings.