Green Security is a new research field defining and investigating security solutions under an energy-aware perspective. Green Security aims at: (1) evaluating the actual security mechanisms in order to assess their energy consumption; (2) building new security mechanisms by considering energy costs from the design phase. In this paper, we first provide a definition of Green Security and formalism to model it, then we provide a use case showing how it is possible to model the energy consumption of two Intrusion Detection System (IDS) strategies, finally we leverage this model to assess the energy leakage due to the late discovery of bad packets.