Water and energy are the main natural resources, and their rational use is the basis for sustainable development. Therefore, the energy efficiency of water supply networks is one of the priorities for the management system of water utilities. Many methods and indicators can be used to assess the energy efficiency of water distribution networks (WDNs), and their choice should be adapted to the characteristics of a WDN. This paper presents an energy audit of WDNs that are supplied from five reservoirs located above the supply area, to which water is supplied from four underground and surface water intakes. In the analysis of the system operation, a hydrodynamic computer model of the water distribution network was used to estimate the operating parameters that are necessary to determine the energy efficiency indicators. A new method for calibrating the emitter coefficient used for water loss modeling is also proposed. The conducted audit showed that more than 70% of the energy supplied to the WDS was “lost”, mainly due to friction (37%) and water losses (27%). Thanks to hydraulic modeling, it was possible to indicate that 34% of the energy lost in the system was related to the use of pressure-reducing valves (PRV), and that only 3% was directly related to friction. In turn, the majority of leaks are attributed to service connections (17.4% vs. 8.8% in the water distribution network).