This lecture covers the concept of instantaneous power in AC circuits, explaining how to calculate power using voltage and current waveforms. It delves into the components of instantaneous power, such as constant and sinusoidal components, and discusses power and reactive power for resistors, inductors, capacitors, and impedances. The lecture also introduces the complex apparent power, represented by S = P + jQ, and discusses the analogy between power units. Various examples are provided to illustrate the calculation of reactive power in different circuit elements.