This article focuses on the key issue of energy dissipation in passive rockfall protection structures when exposed to impact by a rock block. As an application case, a structure consisting of a wall made of interconnected concrete blocks is considered. A Non-Smooth Contact Dynamics (NSCD) model of this structure was previously developed and calibrated with respect to the spatio-temporal impact response obtained from real-scale impact experiments. The NSCD model accounts for energy dissipation due to friction between the system bodies and to plastic strain at contacts. The energy dissipation computation method is detailed and its exactness is demonstrated considering two impact cases. The evolution with time of energy dissipation by each dissipative mechanism provides insights into the global structure response with time in terms of displacement and contact force amplitude. The influence of the model parameters on the contribution of these two dissipative mechanisms is evaluated. A ratio between energy dissipation by friction to energy dissipation by plastic strain is proposed as a criterion for structural response evaluation. The variability in energy dissipation varying the impact conditions is addressed. In the end, this study reveals the benefits derived from a precise quantification of energy dissipation in passive rockfall protection structures. Beyond the considered application case, such an approach focusing on energy dissipation opens up promising prospects for improving the design of all passive rockfall mitigation structures.