Data from direct measurements of X-ray fluxes from flares (1-8 A, GOES) for three solar cycles (1977-2007) were used to calculate integral energies of flares associated with active regions of spot groups of different class. For the flares in active regions of each class we constructed integral energy spectra and calculated their indices В. It was found that the В parameter value differed for active regions of different class. In each class, we revealed a positive correlation between this parameter and 11-year cycle (Wolf number). This result is interest in forecasting the mean flare energy in different phases of solar activity that, in turn, determines the space weather conditions.