The evaluation of the power brought by an electric arc to the surface of the electrodes still remains an important problem. In this paper we propose a method allowing a macroscopic approach of the power balance at the surface of a copper cathode submitted to a non stationary electric arc in argon. This method is based on a spectroscopic measurement of the instant of apparition of copper vapour in the arc and on the use of a numerical modelling of thermal phenomena occurring in the electrode. Under some simplifying assumptions, we reach an interval of possible values for the power and the surface density brought to the electrode by the arc.