Acceleration-based or displacement-based seismic design methods do not incorporate the frequency content, duration or number of ground motion cycles; on the other hand, energy-based seismic design does include the frequency content of the records, which means that there is a difference between the amount of input energy demand for different types of earthquakes, earthquakes as different as those generated in the far-field and those produced in the near-fault. However, this method does not yet take into account the time of the seismic recordings. Since the strong ground motion duration has a significant effect on the magnitude of damage to structures, this paper incorporates this duration through the concept of power demand or input energy rate, and presents three different methods for its quantification: (1) the derivative with respect to time of the energy balance equation; (2) the maximum energy for the instant in which the single-degree-of-freedom elastic oscillator reaches its maximum displacement; and (3) the average speed at which the seismic input energy transferred to the same oscillator. Based on the proposed methodology, the power demand spectrum is correlated with the total amount and time that the single-degree-of-freedom elastoplastic oscillator needs to transform or dissipate the input seismic energy into energy by hysteresis, and it is shown that near-fault earthquakes with impulsive characteristics become earthquakes with low input energy, but highly powerful.