Anomalous energy consumption detection is a valuable strategy for pursuing energy efficiency. In commercial buildings, such as supermarkets, abnormal consumption can occur due to non-adequate equipment, such as lighting devices and refrigeration systems, or non-efficient HVAC plant management. Anomaly detection is usually performed on a single building by comparing its energy consumption to its usual behaviour and applying statistical or artificial intelligence-based techniques. Still, no anomaly emerges if its energy consumption is systematically high (or low). However, a more effective method for detecting anomalies would be to compare the energy consumption of a single building with that of others possessing similar characteristics. This paper then proposes an alternative approach based on clustering analysis. From this perspective, energy consumption data from a group of supermarkets are gathered in clusters to detect which presents abnormal behaviour compared to others with similar characteristics, such as the dimension and external weather conditions. An unsupervised density-based clustering algorithm for outlier detection (DBSCAN) is applied to a pool of 87 supermarkets located in Tuscany (Central Italy) to detect the abnormal ones, considering as input features the floor area, the electrical and thermal consumptions available from monthly bills, the type of the air-conditioning units, and the outdoor temperature. The analysis is performed over three years to detect recurring outliers on an annual and monthly scale to investigate possible seasonal effects. During the three years, approximately 15% of the supermarkets were consistently identified as outliers on both a monthly and annual basis. These findings were subsequently validated through an on-site inspection conducted by the energy manager of the supermarkets, revealing that 50% of the identified outliers exhibited exceptionally high thermal and electrical consumption due to improper plant operation.