This article presents an equivalent circuit approach used for the modelling of a system consisting of an antenna and a RF/DC rectifying circuit (rectenna). This system is developed for microwave energy transfer to low energy consumption wireless devices at 2.45 GHz. The measurements and simulations steps used for parameter extraction and model validation are presented. It is shown that using this approach, no electromagnetic simulation tool is required for the simulation and optimization of the overall circuit. Furthermore, this method allows for the physical origin of the energy losses in the system to be quantified. Results obtained using this simple example show a good agreement with the experiment on a broad frequency range and can form the basis for further developments of more complex systems.