Reactive power
The component of AC power that oscillates between source and load without doing net work, needed to sustain magnetic and electric fields and control voltage.
In AC systems, current and voltage are not always in phase. Reactive power (Q), measured in volt-amperes reactive (var), is the portion that flows back and forth each cycle because of inductance (motors, transformers, overhead lines) or capacitance (cables, capacitor banks). It does no net work but is essential to maintain the magnetic fields of motors and transformers and to control voltage across the network.
Here S is apparent power (VA), P active power (W) and φ the phase angle between voltage and current. The ratio P/S is the power factor.
Voltage on transmission networks is closely linked to reactive power balance: absorbing vars lowers voltage, injecting raises it. Lightly loaded cables and lines generate vars, causing high voltages at night; heavily loaded lines consume vars, risking voltage collapse, a factor in several major blackouts (France 1978, North America 2003). Overvoltage control was also a central issue in the 2025 Iberian blackout investigation.
Reactive power sources include synchronous generators and synchronous condensers, static var compensators (SVCs), STATCOMs, shunt reactors and capacitors, and modern wind, solar and battery inverters. European grid codes (RfG) require new generators to operate across a defined power factor range, commonly about 0.90 to 0.95 lagging to leading. Reactive power is often procured as an ancillary service and must be supplied locally because it travels poorly over distance.
Related terms
All terms →- Power factor
The ratio of active power to apparent power in an AC circuit, between 0 and 1, indicating how effectively current delivers useful work.
- Ancillary services
Services beyond bulk energy that system operators procure to keep the grid stable and secure, such as frequency reserves, voltage control and black start.
- Transmission and distribution losses
Electricity lost as heat and through non-technical causes between power plants and end users, usually expressed as a share of energy supplied.
- Grid frequency
The rate at which alternating current changes direction on a power system, 50 Hz or 60 Hz, which reflects the instantaneous balance of supply and demand.
- HVDC (High-Voltage Direct Current)
Transmission technology that carries bulk power as direct current, with lower losses over long distances and the ability to link asynchronous grids.
- Grid-forming inverter
An inverter that sets its own voltage and frequency reference, behaving like a voltage source that can stabilise weak grids and provide synthetic inertia.