An energy audit is a standard process to support the decision-making in the area of energy management. Nevertheless, an energy auditor does not succeed as easily and quickly in identifying all the energy streams in a facility in order to decide whether retrofitting the audited object is beneficial. In fact, energy auditing of buildings is usually a time consuming and expensive process, due to efforts required for data collection and modelling of audited objects. Therefore, a decision-making tool should support the modelling phase and this purpose might be achieved by means of reduced-order modelling, after a quick data acquisition, through on-site measurements. As highlighted in literature, reduced-order models, also called grey-box models, showed their reliability to achieve a suitable description of the thermal response of buildings in a short time. Besides, they are more cost-effective and their thermal parameters can be usually extracted in short time. Consequently, deriving a lumped parameter model, by means of measurements collected in a rather short period, could allow the owners and managers of real estates to perform fast and cheap preliminary assessment on the opportunity to implement energy renovation actions. Starting from a test performed in the machine laboratory of our department at the Universit? Politecnica delle Marche, the investigated empirical procedure for deriving grey-box models will be provided and energy saving opportunities, within an energy retrofit, will be analysed as an example about the application of this procedure.