The present paper reports the main results of a study the aim of which being to apply the Power Balance Method to the electric field calculation in the ARIANE 5 launcher cavities of the Payloads section and in the launcher avionics bay. Indeed, the use of this simple method, based on the application of the Poynting's theorem and on the statistical properties of electrically large cavities stimulated by high frequency sources, is the most efficient one in terms of computation time and high frequency limit with respect to numerical methods like 3D Finite Differences in the Time Domain for example.