During the synthesis optimisation of an energy system, the configuration changes and there is need to adapt properly the mathematical model of the system. A method is presented here for the automatic synthesis of the model itself of the energy system, which is based on the graph theory. The topology of the graph is stored in the computer memory and the computer model of the respective system is constructed automatically by Object Oriented Programming. The modelling diagram of the system is introduced by an Application Programming Interface. A combined-cycle system serves as an application example. The method has been proved efficient and convenient. (C) 2007 Elsevier Ltd. All rights reserved.