The main goal of the Task 4 “Development of thermal diagnostics of buildings” of Strategic Research Project “Integrated System for Reducing Energy Consumption in the Maintenance of Buildings” was to develop new tools for thermal diagnostics of different types of buildings, including development of the rapid method of a cooling sources diagnostics. The paper presents a method for assessing the cooling sources based on on-site measurements. The cooling capacity of the cooling sources and the electric power consumed by the cooling sources were measured in two air-conditioned office buildings. The measurement results were used to calculate the cooling energy generated during the measurement period and the operational Energy Efficiency Ratio