The theoretical value of heat of formation of benzene is 288 kJ/mol. But the observed or experimental value is 83 kJ/mol. As both of these values are positive, it is evident that the real benzene molecule is (288-83) = 205 kJ/mol more stable than the calculated value based on a single Kekule structure. This is due to resonance stabilization arising out of delocalization of labile pi elctrons through all possible equivalent structures of benzene (both Kekule type and Dewar type). This difference in energy is known as resonance energy of benzene. Greater the extent of delocalization, better is the stabilization and higher is the resonance energy.