Using a first-principles approach, we assess the validity of a picture for the energetics at Si-SiO2 interfaces based on bond energies complemented with penalty energies for silicon atoms in intermediate oxidation states. By total-energy calculations on cluster models, we demonstrate that such penalty energies only depend on the composition of the first-neighbor shell of the silicon atoms and can thus be taken as additive contributions to the total energy. Considering oxygen incorporation processes in Si-SiO2 interface models, we show that variations in the interface energy result from suboxide and strain contributions of comparable magnitude. Hence, simplified schemes for the energetics at Si-SiO2 interfaces should account for both contributions with similar accuracy.