The instantaneous power in an AC circuit is given by: p = i(t)⋅v(t) = VrmsIrmscosφ + VrmsIrmscos(2ωt -φ). The average power P = VrmsIrmscosφ is often a useful quantity to know. For example, it can tell me the work being done by a motor. The apparent power S = VrmsIrms is also a useful quantity to know. For example, it can tell me the maximum work the motor could potentially do (when the current and voltage are in phase φ=0). However... the reactive power Q = VrmsIrmssinφ = sqrt(S2 - P2) doesn't really tell me anything useful. All I know is that it is some unintuitive measure of the difference between the apparent and average power. Why isn't reactive power simply defined as Q = S - P instead?