Energy consumption for space heating and cooling of buildings can be decreased by 40-80% by use of Aquifer Thermal Energy Storage (ATES). ATES is a proven technique, however, it is not known how efficient currently operating systems are recovering stored energy from the subsurface and how this can be determined with available data. Recent research suggests that storage conditions have a large influence on the recovery (e.g. shape and size of stored volume). In addition, literature and previous research show that other aspects of ATES system are often unfavorable (e.g. subsurface energy imbalance, small ΔT). Therefore, the main goal of this research is to define a framework to determine overall performance of ATES systems by analysis of monitoring data from operational ATES systems. The province of Utrecht was selected for this. Monthly operational data of 57 ATES systems (40% of the ATES systems in the province) was provided by the authorities and pre-processed accordingly. Results showed that recovery efficiency is positively correlated to system size (stored volume) and that ambient groundwater temperature is site-specific and should be determined for each ATES system individually. Ambient groundwater temperature can vary more than 4 °C and are spatially correlated. Next to this, a large part of the analyzed systems are not equally storing heat and cold in the subsurface. More than 80% of the studied ATES systems have an subsurface heat imbalance larger than 10%. Altogether, results indicate that a big part of the ATES systems in the province of Utrecht can substantially improve their ATES system (management) to increase long-term energy savings. This research provides an useful assessment framework to determine if an ATES system is performing correctly and what aspect of the specific ATES system needs most improvement.