The wastewater sector accounts for 25% of the global energy demand in the water sector. Since this consumption is expected to increase in the forthcoming years, energy optimization strategies are needed. A truly effective planning of energy improvement measures requires a detailed knowledge of a system, which can only be achieved through energy audit and real-time monitoring. In order to improve the identification of critical issues related to the use of energy resources within a wastewater treatment plant (WWTP), the paper shows the results of a monitoring campaign performed on a large WWTP in southern Italy. Data obtained for the audit cover a 4-year timeframe (2014-2017). Energy-environmental performance has been evaluated through the benchmarking of: system variables, specific consumptions, and operational indicators. Moreover, by using a real-time data measurement and acquisition system it has been possible to evaluate the real performance of the most energy-intensive apparatus of the plant (a turbo-blower), over a period of 8 months. The main results indicate that (a) the plant is mainly affected by a massive capture of infiltrations, working in conditions close to the maximum hydraulic capacity, (b) real-time energy measurements are necessary to accurately characterize plant consumptions and adequately assess their critical aspects.