||Water and energy are two inextricably linked resources. Each has the potential to limit the development of the other. There is a substantial body of research dedicated to understanding how the availability of water can limit energy production, but the alternate relationship - that of energy limiting water production - has received much less scrutiny. The demand for both resources is predicted to increase in tandem with population growth, potentially creating or adding to conflict in regions of water or energy scarcity. To greater understand the "water/energy nexus," - a commonly used term to describe their interdependence - each phase of water supply and consumption can be broken into discrete segments that have an associated energy requirement, called an energy factor. An energy factor is the amount of energy used to develop, convey and treat a given volume of water. This study presents a methodology for calculating the energy factors of each phase of the water supply cycle that is "outside the retail meter." A case study of a large water system in an arid region of the United States is used as an example system for applying these methods. Using the case study system as a framework, an energy demand model is developed that estimates baseline energy usage for heterogeneous water systems, and then models changes in energy requirement under three alternate water supply and demand scenarios. The results of the model scenarios reveal that water demand reductions, as can be brought about by targeted water efficiency programs, can have extended energy-saving impacts - affecting all other phases of the water supply cycle. A demand reduction of 25% for the case study water system resulted in a cumulative annual energy savings of 8.9 million kilowatt hours (kWh) - a decrease of 28% from its current level of energy consumption. Modeling the conversion of agricultural or currently untreated water to municipal uses within the case study resulted in an increase in energy requirement by 6.3 million kWh - a 20% increase. Reductions in the availability of imported surface water supply, such as those brought about by prolonged drought, climate change or reservoir sedimentation, can increase energy demand as well. An additional 5.7 million kWh are needed to ameliorate the effects of a 35% reduction in surface water supply for the case study water system - an 18% increase from its current energy requirement. The process and findings of this study reveal a lack of emphasis among water agencies concerning energy consumption, and indicate that changes in supply and use patterns have dramatic effects on energy usage.