The principle of air compression can be utilized for the storage of electrical energy, with variable properties from the point of view of the energy efficiency. For the involved thermodynamic phenomena different approaches and realized systems lead to more or less acceptable results. In the hybrid system studied by the LEI, Laboratoire d'?lectronique Industrielle of EPFL, specific strategies have been selected, in the sense of using variable quantities and degrees of freedom in order to get good energetic performance. This choice has the consequence of using multiple conversions by power electronic circuits. In the complete "round-trip" cycle of the energy storage, these conversions have a great influence on the resultant efficiency. An analytic calculation of the global efficiency of such a system is presented in this contribution.