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It is the average amount of work done or energy converted per unit of time. Average power is often called "power" when the context makes it clear. Instantaneous power is the limiting value of the average power as the time interval Δt approaches zero.
The portion of instantaneous power that, averaged over a complete cycle of the AC waveform, results in net transfer of energy in one direction is known as instantaneous active power, and its time average is known as active power or real power.
Instantaneous power is still the product of instantaneous current and instantaneous voltage, but if both of those are ideal sine waves driving a purely resistive load (like an incandescent light bulb), average power becomes (with subscripts designating average (av), peak amplitude (pk) and root mean square (rms)):
The plotted line represents the variation of instantaneous voltage (or current) with respect to time. This cycle repeats with a frequency that depends on the power system. In electrical engineering, three-phase electric power systems have at least three conductors carrying alternating voltages that are offset in time by one-third of the period ...
The time-averaged power flow (according to the instantaneous Poynting vector averaged over a full cycle, for instance) is then given by the real part of S m. The imaginary part is usually ignored, however, it signifies "reactive power" such as the interference due to a standing wave or the near field of an antenna.
Instantaneous and average power calculated from AC voltage and current for a load with a lagging power factor (ϕ = 45°, cos(ϕ) ≈ 0.71). The blue line (instantaneous power) shows that a portion of the energy received by the load is returned to the grid during the part of the cycle labeled ϕ.
Unlike average power, which is the mean power output over a period, peak power represents the maximum power output at any given instant. This distinction is crucial in applications where signal peaks can significantly exceed the average power level. Peak power is a critical parameter in the field of radio frequency (RF) and telecommunications.
More commonly used is the power spectral density (PSD, or simply power spectrum), which applies to signals existing over all time, or over a time period large enough (especially in relation to the duration of a measurement) that it could as well have been over an infinite time interval. The PSD then refers to the spectral energy distribution ...