Performance assessment is essential to effectively evaluate and monitor the activity of water utilities, support decision making, and encourage continuous improvement. Performance assessment systems (PAS), covering several service objectives and criteria, have been successfully applied in water supply and wastewater systems. Tailored approaches focusing on the assessment of the energy use and efficiency in wastewater systems are still limited. This paper aims at the development and demonstration of a comprehensive PAS for energy efficiency, tailored for wastewater systems, incorporating criteria related to energy consumption, operation and maintenance (O&M) costs, and environmental impacts, such as untreated discharges and greenhouse gases emissions, among others. Management and control of excessive or undue inflows to these systems is specifically addressed by several novel criteria and metrics. The proposed PAS should be adapted by each utility to be aligned with the objectives of the organisation and with the implemented asset management strategy. The proposed approach and the resulting consolidated PAS are thoroughly described. Results from the application of the PAS to several Portuguese utilities are discussed. This PAS aims at contributing to a reliable and replicable process to assess energy efficiency in wastewater systems and to encourage a more rational energy management.