Given the increasing importance of the wastewater sector in terms of energy usage, the understanding of the level of energy efficiency of wastewater treatment plants (WWTPs) is useful to both the industry itself as well as policy makers. Here, based on economic foundations, we apply a Stochastic Frontier Analysis (SFA) approach for energy demand modelling to estimate energy efficiency in the wastewater sector. Using specific SFA models and panel data from 183 Swiss WWTPs over the period 2001 to 2015, the paper illustrates that distinguishing between persistent and transient inefficiency is essential to deduce appropriate energy efficiency diagnosis in WWTPs. In this respect, persistent energy inefficiency is found to be more severe than transient energy inefficiency. Furthermore, it is shown that the age of the equipment influences the demand for energy and the energy savings due to technological innovation are quantified. Finally, economies of output density and scale are estimated demonstrating that for plants operating below optimal scale significant energy savings can be achieved if plants would be operated at higher size. Moreover, our analysis reveals also that for plants larger than 100,000 Population Equivalent, at least from an energy efficiency point of view, it would be no more beneficial to increase their scale.