Reactive power is the portion of electrical power that oscillates between the grid and electrical equipment without being converted into useful work. It is measured in kilovolt-amperes reactive (kVAr). Reactive power is required to create and sustain electric and magnetic fields in equipment such as motors, transformers, and many power systems, but it increases current flow and affects how heavily cables, switchgear, and transformers are loaded.
Why Reactive Power Matters in EV Charging
Even though EV charging is primarily an active power (kW) load, charging sites operate within electrical infrastructure that is affected by reactive power and power factor.
– Influences power factor (PF) and therefore the total current drawn from the grid
– Affects apparent power (kVA) loading of transformers and switchboards, which can limit expansion
– Increases cable losses and heating due to higher current for the same delivered kW
– Can trigger utility penalties or requirements in a grid connection agreement
– Impacts voltage stability in constrained sites with long feeders and high simultaneous load
Reactive Power vs Active Power vs Apparent Power
These three quantities describe different aspects of AC power flow.
– Active power (kW): the useful power that actually charges the EV battery and runs equipment
– Reactive power (kVAr): field-related power that flows back and forth and does not do net work
– Apparent power (kVA): the total demand seen by the grid (kVA ≈ √(kW² + kVAr²))
A higher kVAr value generally means higher kVA for the same kW, increasing infrastructure loading.
What Creates Reactive Power
Reactive power arises mainly from inductive or capacitive behavior in AC systems.
– Inductive loads (motors, transformers) typically draw reactive power (lagging PF)
– Capacitive elements (capacitor banks, long cables) can supply reactive power (leading PF)
– Power electronics can affect PF and harmonic distortion, influencing how reactive behavior is measured and managed
Reactive Power in EV Charging Sites
Reactive power at a charging location can come from both charging equipment and the wider site.
– Building loads such as HVAC, lifts, ventilation fans, and pumps
– Transformers and long feeder cables supplying charging distribution boards
– Certain charger designs and filters, especially in high-power installations
– On-site generation and inverters (solar PV, storage) depending on operating mode and settings
Power Factor and Utility Impacts
Reactive power is closely tied to power factor, which affects billing and compliance.
– Low PF increases current and may lead to reactive energy charges or penalties
– Utilities may require PF to stay within a defined band at the point of connection
– Managing kVAr improves capacity utilization and can reduce operating cost
How Reactive Power Is Managed
Common mitigation methods aim to keep PF within target limits and stabilize voltage.
– Reactive power compensation using capacitor banks or APFC systems
– Detuned capacitor banks to reduce resonance risk in harmonic environments
– STATCOMs or active compensators for dynamic reactive power control
– Power quality studies to ensure stable operation with chargers, PV, and storage
Related Glossary Terms
– Power factor (PF)
– Reactive compensation
– Reactive power compensation
– Apparent power (kVA)
– Power quality study
– Harmonics
– Grid connection agreement
– Voltage drop