A simple method is proposed to calibrate the energy of an electron beam. An electrical floating metal foil is irradiated by the incident electrons. The electron energy is deduced from the magnitude of the surface potential developed on the irradiated conductor surface. The measurement of the surface potential is made outside the vacuum chamber on a metallic slab connected through a coaxial feedthrough to the electron irradiated sample. The energy calibration obtained by this method is compared to the conventional method based on measuring the energy position of the backscattered electron elastic peak.