The energy required to supply one cubic meter of tap water in a conventional water supply system comprises of the energy required to water intake, transmission, purification and distribution. In a conventional water supply system, the energy requirement depends on the quality of raw water, distance to the user, and elevation difference between the source and the user. In this paper, the unit energy requirement for water supply system (energy required to supply one m³ tap water) is calculated using the average value considering an arbitrary city. The distance of distribution is assumed from 100 m to 10,000 m. The average elevation is also assumed from 0 m to 100 m. Alternative water supply systems such as rainwater harvesting system and grey water system are calculated and evaluated. These data can be used by the decision makes and policy makers to decide the optimum water supply solution for the sustainable growth in preparation for the climate change. Especially, it is noted that the energy saving by using rainwater harvesting is tremendous, which will be discussed with modelling and calculation results.