Resonance energy = actual enthalpy of formation - calculated enthalpy (assuming no resonance). For benzene: calculated $delta_{Hc}$ assuming 3 C=C and 3 C-C bonds gives a more negative value than observed. The difference (approximately -150 kJ/mol for benzene) is the resonance stabilisation energy. Benzene is more stable than the hypothetical cyclohexatriene. Resonance energy can also be estimated from hydrogenation enthalpies: cyclohexene has delta_H_{hyd} = -120 kJ. Expected for 3 double bonds: -360 kJ. Actual benzene delta_H_{hyd} = -208 kJ. Resonance energy = 360 - 208 = 152 kJ/mol.