In this paper, I propose a new equation describing the evolution of seismic sequences, based on radiated energy. The evolution of radiated energy in time is described as function of the p and c parameters of the modified Omori law and of the energy radiated within a short time following the mainshock. By using the energy rather than the number of events in describing seismic sequences, I circumvent the problem of missing weak aftershocks close in time to the mainshock, because most of the total energy is contained in largest events. In addition, I propose an equation describing the difference in magnitude between the mainshock and the strongest aftershock as function of the energy radiated in a short time after the mainshock, of p, c, and of the parameter b of the Gutenberg Richter equation. An application to California sequences shows values of p in the range [0.65, 1.5] and c in the range [0, 0.25] and no dependence of c on the difference between mainshock and cutoff magnitude.