A reactive volt ampere (VAR) is the unit of measurement for reactive power, representing the energy that continuously sloshes back and forth between the AC source and the magnetic or electric fields of a load without performing actual useful work. If you are dealing with inductive loads like AC motors, transformers, or solenoids, you are generating VARs. While the utility meter doesn't spin backward to credit you for the returned energy, the physical wires and breakers in your facility must be sized to carry the total current this sloshing creates.
The Power Triangle and the Water Hose Analogy
To understand what a reactive volt ampere actually does, we have to look at the power triangle. In an AC circuit, voltage and current are not always perfectly in phase. When current lags voltage (inductive loads), you get reactive power.
According to the fundamentals outlined in All About Circuits' AC theory textbook, the relationship is defined by the Pythagorean theorem: $S^2 = P^2 + Q^2$. Apparent power ($S$, measured in VA) is the hypotenuse. Real power ($P$, measured in Watts) is the horizontal leg. Reactive power ($Q$, measured in VARs) is the vertical leg.
What VARs Actually Change in Your Installation
Reactive power does not change the amount of useful work your motor performs, but it fundamentally changes the current draw and the thermal loading of your electrical distribution system. Let's look at a concrete numeric example to see how this impacts wire sizing and breaker selection.
Worked Numeric Example: 480V 3-Phase Motor
Assume you have an industrial load drawing exactly 6,000 Watts of real power at a lagging power factor (PF) of 0.80, supplied by a 480V 3-phase source.
- Apparent Power (S): $6000W / 0.80 = 7,500 VA$ (or 7.5 kVA).
- Reactive Power (Q): $\sqrt{7500^2 - 6000^2} = 4,500 VAR$ (or 4.5 kVAR).
- Total Line Current: $7500 VA / (480V \times \sqrt{3}) = 9.02 Amps$.
If you could magically force the power factor to 1.0 (eliminating the 4.5 kVAR), the current required to deliver that exact same 6,000 Watts would drop to 7.21 Amps.
That extra 1.81 Amps of "sloshing" current does zero useful work, but it causes $I^2R$ heating in your conductors. If you are running 10 AWG THHN in a conduit with three other current-carrying conductors, that extra thermal load accelerates insulation degradation. Furthermore, as noted in Fluke's guide on power quality, utilities often penalize commercial facilities with power factors below 0.95, either by billing directly for kVARh or by charging for peak kVA demand instead of kW demand.
Where You Meet VARs in Practice
You won't usually measure VARs on a standard $20 multimeter, but you will encounter their effects in several specific scenarios:
- Industrial Motor Plants: Facilities with hundreds of lightly loaded induction motors. The magnetic fields required to run these motors draw massive lagging VARs, tanking the facility's power factor and triggering utility penalty clauses on the monthly electric bill.
- Solar Inverters and Grid Support: Modern string inverters (like the SMA Sunny Tripower or Fronius Symo) can be programmed to inject or absorb VARs. This is known as "VAR support" or Volt/VAR control, which helps the utility stabilize local grid voltage without altering the real power (Watt) export.
- Welding Transformers: Older stick welders are highly inductive. When you strike an arc, the massive inrush of reactive current can cause severe voltage sags (brownouts) on the branch circuit, dimming lights and tripping sensitive electronics.
Decision Tree: Sizing Capacitors to Kill Excess VARs
To eliminate lagging VARs, you install power factor correction (PFC) capacitors in parallel with the load. Capacitors draw leading reactive current, which perfectly cancels out the lagging reactive current of the inductive load. Use the decision matrix below to determine your exact next step.
| Condition / Measurement | Action Required | Hardware / Next Step |
|---|---|---|
| PF is > 0.95 and utility bill has no kVAR penalty. | Do nothing. System is optimized. | N/A |
| PF is < 0.95, but load is highly variable (e.g., CNC machines cycling on/off). | Install an automatic PFC controller bank at the main service entrance. | LOVATO DCRG Automatic Power Factor Controller with switched capacitor steps. |
| PF is < 0.95, driven by a single large, continuously running motor (e.g., 50HP+ fan). | Calculate missing kVAR and install a fixed capacitor directly at the motor starter. | Proceed to sizing calculation below. |
| Harmonics (THDv) are > 5% due to VFDs or LED drivers. | Do NOT install standard capacitors; they will resonate and explode. | Use detuned (anti-harmonic) reactors in series with the capacitors. |
Concrete Sizing and Part Selection
Returning to our numeric example: we have a continuous load generating 4.5 kVAR of reactive power at 480V. We want to correct this to a 1.0 power factor locally at the motor starter.
Standard capacitor sizes step in increments of 2.5, 5, 7.5, and 10 kVAR. You should never overcorrect (which creates a leading power factor and can cause dangerous overvoltage conditions at the motor terminals when it disconnects). Therefore, you round to the nearest standard size that does not exceed the motor's no-load reactive draw. A 5 kVAR capacitor is the standard engineering pick here.
Installation Safety Warning: Working with PFC capacitors involves lethal mains voltage and stored energy. De-energize the panel, apply lockout/tagout, and verify dead with a tested CAT III/IV meter. Crucially, even when de-energized, a capacitor can hold a lethal charge. Always use a properly rated grounded discharge stick to short the capacitor terminals to ground before touching any busbars or wiring.
Frequently Asked Questions About Reactive Power
Does my residential utility charge me for VARs?
No. Residential meters (like the standard Landis+Gyr or Itron smart meters installed by US utilities) only measure and bill for real energy consumed in kilowatt-hours (kWh). The utility absorbs the cost of residential reactive power losses in their overall rate structures. You only need to worry about VAR billing if you are on a commercial/industrial demand rate.
Can I use a 240V capacitor on a 480V system?
Absolutely not. Capacitive reactive power scales with the square of the voltage ($Q = V^2 / X_c$). If you wire a 240V-rated capacitor to a 480V line, it will attempt to generate four times its rated kVAR. The dielectric insulation inside the can will fail catastrophically, resulting in a venting fire or an explosion. Always match the capacitor voltage rating to the system line-to-line voltage.
Why do solar inverters care about VARs if they only produce DC-to-AC Watts?
Modern grid-tied inverters have excess current capacity during non-peak sun hours. Utilities use smart inverters to inject or absorb VARs to regulate grid voltage. If the local grid voltage sags, the inverter injects leading VARs to prop it up (Volt/VAR curve). This is a requirement under IEEE 1547-2018 interconnection standards for most new commercial solar installs.
