What exactly are active and reactive power, as often mentioned by energy storage companies?

This article breaks down the definitions, differences, and functions clearly:

⚡ What is Active Power?

Active power is the power that directly performs work and is converted into mechanical energy/heat energy/light energy. It is the energy that the load “actually uses,” and its core function is to maintain a stable system frequency (50Hz).

For example, the rotation of a motor and the illumination of a light bulb consume active power.

🔋 What is Reactive Power?

Reactive power does not directly perform work; it is only used to establish and maintain the magnetic field of electrical equipment. It is not consumed, but the equipment cannot operate without it. Its core function is to maintain a stable system voltage (220V).

📌 Key Differences

Active power: One-way energy consumption, determining the grid frequency;

Reactive power: Circulating energy exchange, determining the grid voltage;

The combination of both is key to the stable operation of the power grid.

The power output of a generator is divided into two forms: active power and reactive power. The direction of this power flow determines the generator’s operating mode: If the generator outputs active power, it operates in generator mode; if it absorbs active power, it switches to motor mode, at which point reverse power protection will activate. If the generator outputs inductive reactive power, it operates in a lagging phase; if it absorbs reactive power from the system (or outputs capacitive reactive power), it operates in a leading phase.

Essentially, the core definitions are as follows: Active Power (P): This refers to power that can directly perform work and be converted into usable energy forms such as mechanical energy, heat energy, and light energy. It is the energy that the load “truly needs” to operate, and its unit is watts (W). For example, the power consumed by a motor rotating or a light bulb emitting light is active power.

Reactive Power (O): This refers to power that does not directly perform work but is used only to establish and maintain the magnetic field required for the operation of electrical equipment such as motors and transformers, and its unit is volt-amperes (Var). It only exchanges power periodically between the generator and the load and is not consumed, but it is a necessary condition for the normal operation of electrical equipment. Without reactive power, the motor cannot generate the magnetic field to drive the rotor and cannot rotate at all.

The generator rotor receives mechanical energy from a prime mover such as a steam turbine or water turbine. The rotor’s magnetic field interacts electromagnetically with the stator windings, unidirectionally converting mechanical energy into electrical energy, which is then transmitted to the load and converted into various types of useful work. This is a “energy-consuming” unidirectional conversion process.

Reactive Power Generation: When current flows through the generator stator windings, an alternating magnetic field is generated. The energy of this magnetic field is periodically exchanged with the magnetic field energy of inductive loads such as motors (load absorbs/releases energy – generator absorbs/releases energy). The entire process has no energy loss and is a “energy-exchange” cyclic process.

(I) Factors Affecting Active Power (P): Prime Mover Input Power

This is the core factor determining active power output. For example, increasing the steam output of a steam turbine or the water intake of a water turbine will increase the rotor speed, thereby increasing active power output; conversely, active power output will decrease.

System Active Load: Changes in the active power demand of loads such as factory motor operation and residential daily electricity consumption will directly affect the active power output of the generator. The active power supply and demand balance of the system needs to be matched by adjusting the input power of the prime mover.

(II) Factors Affecting Reactive Power (O):

Generator Excitation Current: This is a key parameter controlling reactive power output. When the excitation current increases, the magnetic field strength generated by the stator winding increases, and reactive power output increases accordingly; when the excitation current decreases, reactive power output decreases, and may even turn into reactive power absorption from the system.

System Voltage Level: When the system voltage is low, the generator needs to increase reactive power output to support the voltage recovery to the normal range; when the system voltage is high, reactive power output needs to be reduced, or even reactive power absorbed to lower the voltage. Load inductance: The more inductive loads such as motors and transformers there are, the greater the demand for reactive power. The generator needs to increase its reactive power output accordingly to meet the load’s operating requirements.

Active power’s primary function is to maintain a stable power system frequency (the standard frequency of my country’s power system is 50Hz).

When active power supply exceeds demand (generator output exceeds load demand), the system frequency increases; when supply falls short of demand (generator output falls short of load demand), the system frequency decreases. Significant frequency fluctuations can lead to abnormal motor speeds, damage to electronic equipment, and in severe cases, even power system collapse.

Reactive power’s primary function is to maintain a stable power system voltage (e.g., the standard voltage for residential electricity is 220V).

When reactive power supply falls short of demand (generator output cannot meet load demand), the system voltage decreases (e.g., light bulbs dim during peak electricity consumption in remote areas); when supply exceeds demand, the system voltage increases. Low voltage can prevent motors from starting, reduce equipment efficiency, and in severe cases, cause a voltage collapse.