The impact energy of an avalanche on the flat surface of an instrumented structure has been quantified by full-scale experiments performed at the Lautaret avalanche test site, France. The deformation and acceleration of the structure were measured during the avalanche. The impact energy of the avalanche was calculated from an energy balance via the mechanical energy theorem. First the elastic potential energy and kinetic energy of the structure were calculated from the measured deformations and an adequate mechanical model of the structure. Internal energy dissipation due to material damping and potential plastic deformation was calculated. Finally the mechanical work done on the structure by the avalanche forces was deduced from the energy balance. Results show that the elastic energy is the main component of the energy injected in the structure. The kinetic and viscous components are negligible because of the very low displacement rate of the structure during the impact. Another important result is that the maximum of the mechanical power (work rate) is done when the head of the avalanche impacts the structure. This occurs before the pressure reaches its nominal value, approximately when the product of the pressure by the pressure rate is maximum.