An article in Journal of Water Resources Planning and Management quantifies the energy savings that result from water conservation.

Saving water saves energy. Consequently, implementing integrated water management (IWM) measures that reduce potable water consumption, stormwater runoff, and wastewater generation can potentially translate into significant energy savings. In this paper, the energy savings associated with IWM measures of rainwater harvesting and gray-water reuse are estimated, both at national and local utility scales using published data. At the national scale, it is estimated in this paper that up to 3.8  billion kWh and $270 million can potentially be saved annually by replacing landscape irrigation and other outdoor water uses through rainwater harvesting alone, and up to 14  billion kWh and $950 million in combination with gray-water reuse. Similarly, in Charlotte, North Carolina, the local water utility can potentially save up to 31  million kWh and $1.8 million annually. However, annual energy and associated cost savings per household are low at either scale, ranging between 1 and 120 kWh with associated cost savings of less than $10. These results are discussed in terms of energy savings’ role in IWM policy considerations and promotion of sustainable water use in urban areas.