The concept of reactive power comes into play in AC (Alternating Current) systems, not in DC (Direct Current) systems. This has to do with the nature of AC and DC power and the types of loads they typically serve.
Reactive power (often represented by the variable Q and measured in Volt-Amps Reactive, or VAR) is the power “used” or “absorbed” by inductive and capacitive loads, such as motors and transformers (inductive) or capacitors (capacitive). These devices create a phase shift between voltage and current in an AC circuit, meaning the current and voltage waveforms are not perfectly aligned in time. The amount of power that is out of phase is the reactive power. It is important in the design and operation of AC power systems because it affects the system’s overall efficiency and power factor.
In a DC circuit, current only flows in one direction and does not change phase. That is, there is no time-varying aspect to the current or voltage in a DC circuit, so there is no possibility for a phase shift to occur between voltage and current. DC power is direct and constant. The current in a DC circuit doesn’t have a phase angle, it doesn’t oscillate, and thus there is no reactive power in DC systems.
Simply put, the concept of reactive power doesn’t apply to DC systems because they lack the alternating, phase-shifting characteristic of AC systems that gives rise to reactive power in the first place. In a DC system, all power supplied is “real” or “active” power (often represented by the variable P and measured in Watts), which is the power that actually does useful work.