What Is Reactive Power, and Why Does the Grid Care?

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What Is Reactive Power, and Why Does the Grid Care?

In this guide we follow power around one motor circuit, with animated diagrams, and see why reactive power does no work but still matters.

Source M Motor Active power P: flows one way and does work Reactive power Q: swings back and forth, no net work The wire carries both, so the source must supply the total. 1 · Source sends 2 · Motor does work 3 · Coil swaps energy 4 · Wire carries both
The whole story in a loop. Green dots are active power, going one way to do work. Amber dots are reactive power, swinging back and forth. The wire has to carry both.

Quick answer: Reactive power is the part of the power that moves back and forth between the source and the load without doing any net work. We measure it in VAR (volt-ampere reactive). Coils in motors and transformers need it to build their magnetic fields. They borrow energy for a quarter of a cycle and give it back in the next quarter. But the current still flows in the wires. So it uses up wire capacity and it changes the voltage. That is why engineers must manage it.

The problem: not all current does work

A motor takes electricity from the grid. Most of it turns the shaft. We call this useful part active power, written P. We measure it in watts (W). It ends up as motion, heat or light.

But the motor also draws a second kind of current. This current builds the magnetic field inside the motor. It does no work. So why does the grid carry it at all?

To find out, we compare voltage and current over time. Voltage is the electrical push. Current is the flow. One full up-and-down of the AC wave is one cycle. In a heater, the flow rises and falls exactly with the push. In a motor coil, the flow arrives late. We say the current lags the voltage, here by a quarter of a cycle.

A heater (resistor) voltage and current: in step power = voltage × current Power never goes below zero: all of it works. time → A motor coil (inductor) voltage and current: current lags power = voltage × current Power goes up, then down: the average is zero.
Left: in a heater, current follows voltage, so power (green) is always above zero. Right: in a coil, the current lags by a quarter cycle. Power (amber) goes above zero, then below it by the same amount. The average over a full cycle is zero. That back-and-forth part is reactive power.
Think of it like this

Push on a spring. You give it energy as it compresses. Let go, and it pushes back and returns that energy. Over time, your arm did no net work. But your arm still pushed the whole time. A coil is like that spring. The source pushes, the coil returns, and the wire feels every push.

What is inside: a coil and an iron core

Motors, transformers and long power lines all use coils. A coil is a copper wire wound in many turns, often around an iron core. Current in the coil makes a magnetic field. The iron core guides that field and makes it strong.

A motor needs this field to work. The field is how the supply passes energy to the rotor. But a magnetic field also stores energy. When the current grows, the field grows and takes energy in. When the current falls, the field collapses and gives that energy back.

AC supply pushes current in Copper coil many turns of wire Iron core guides the field Magnetic field stores energy, then returns it
A coil on an iron core. Current in the copper turns makes a magnetic field (blue loop) that runs around the core. Building the field takes energy from the supply. Collapsing it gives the energy back.

The core idea: energy borrowed, then returned

Now we can say what reactive power is. In one quarter of a cycle, the coil takes energy from the source and stores it in the field. In the next quarter, the field collapses and returns that energy. The coil borrows and gives back, again and again.

This borrowed-and-returned flow is reactive power, written Q. Its unit is the VAR, short for volt-ampere reactive. We use the name VAR to keep it apart from the watt. Large values are written in kvar or Mvar.

The swap happens twice in every cycle. On a 50 Hz grid, that is 100 swaps each second. During every swap, real current flows in the wire. Real wire has resistance, so it heats up. So even though the average work is zero, the grid still pays for carrying Q.

The power triangle

Engineers keep three powers apart. Active power P, in watts, does the work. Reactive power Q, in VAR, is the swap. Apparent power S, in volt-amperes (VA), is the total the source must supply.

They do not add like plain numbers. They meet at a right angle, like the sides of a triangle. So S² = P² + Q². The angle between P and S is φ (phi). The power factor is cos φ, which equals P ÷ S. It tells us what share of the total does real work.

φ Active power P = 100 kW Reactive power Q = 75 kvar Apparent power S = 125 kVA Check S² = P² + Q² 125² = 100² + 75² Power factor 100 ÷ 125 = 0.8
The power triangle for a motor that draws 100 kW at a power factor of 0.8. P and Q sit at a right angle. The source must supply the long side, S. A smaller angle φ means a better power factor.
Think of it like this

Picture a mug of beer. The liquid is the active power, the part you want. The foam is the part you cannot drink, but it still takes space. The whole mug is the apparent power. Too much foam means a bigger mug for the same amount of beer. Too much reactive power means bigger wires and transformers for the same real work.

Power factor 1.0 Mug = S = 100 kVA Beer = P = 100 kW No foam. Power factor 0.8 Mug = S = 125 kVA Beer = P = 100 kW Foam = extra size.
The mug is a rough picture, not exact maths. Both mugs hold the same 100 of beer. At power factor 0.8, the foam forces a mug that is 25% bigger.
Common mistake

Reactive power is not lost energy. It is not burned or used up. It comes back every cycle. The cost is the current it forces through the wires, which still heats them and still fills their capacity.

Step by step: one cycle of a coil

Let us follow a coil through one cycle. Each card shows one moment.

power flows in,field grows

1Power flows in

The voltage pushes. The current builds up slowly. The field grows and takes energy from the source.

field is full,energy stored

2The field is full

The current reaches its peak. The field holds its largest amount of energy. At this moment the voltage is passing through zero.

field shrinks,energy returns

3Power flows back

The current falls. The field collapses and pushes its energy back into the source. Nothing was used up.

100× everysecond (50 Hz)

4The swap repeats

In the next half cycle, the same thing happens with reversed polarity. In total, the swap runs twice per cycle.

Who absorbs it and who supplies it

Reactive power has two opposite flavours. This is what lets engineers control it.

Inductive loads absorb reactive power. Motors, transformers and long overhead lines store energy in magnetic fields. Their current lags the voltage. We say they absorb, or consume, lagging reactive power.

Capacitors supply reactive power. A capacitor stores energy in an electric field instead. Its current leads the voltage, the opposite of a coil. So it supplies reactive power. When a capacitor sits next to a motor, the two swap energy with each other. Less has to travel down the line from the far-away generator.

Think of it like this

Two neighbours keep borrowing a ladder from each other. If they live across the fence, the swap is quick. If one lives in another town, every swap means a long drive. A capacitor placed at the motor puts the neighbours next door.

Without a capacitor M Reactive power makes the whole trip, every time. active P reactive Q With a capacitor at the motor M capacitor Reactive power swings only between capacitor and motor. active P reactive Q
Left: the motor’s reactive power travels to the source and back. Right: a capacitor next to the motor takes over the swap. The long line now carries mostly active power.

Fixing it: a worked example

Engineers often add capacitors near big motors to raise the power factor. This is called power factor correction. A common goal is a power factor of about 0.95. Many utilities charge large customers extra when the power factor falls below a set limit, so check your tariff.

Take a motor load of 100 kW on a 400 V three-phase supply, at a power factor of 0.8. We want to raise it to 0.95.

Before: S = 100 ÷ 0.8 = 125 kVA. Then Q = √(125² − 100²) = 75 kvar. The line current is I = S ÷ (√3 × 400 V) ≈ 180 A.

After: at a power factor of 0.95, Q = 100 × tan(arccos 0.95) ≈ 32.9 kvar. So the capacitor must supply 75 − 32.9 ≈ 42.1 kvar. Now S ≈ 105.3 kVA and the current falls to about 152 A. Heat in the wire goes with current squared, so it drops by about 29%. The motor still does exactly the same 100 kW of work.

Reactive power, beforeat power factor 0.8 Capacitor suppliespower factor correction Reactive power, afterat power factor 0.95 Apparent power, beforewhat the source supplies Apparent power, after125 → 105.3 kVA 75 kvar42.1 kvar32.9 kvar125 kVA105.3 kVA
The same 100 kW of work, before and after correction. The capacitor (green) supplies 42.1 kvar locally. The source now supplies 105.3 kVA instead of 125 kVA.

Now try it yourself. Move the slider to change the power factor of the same 100 kW load.

Try it · power factor of a 100 kW load
low power factor zone 0.700.800.901.00 140160180200 Power factor → Line current (A) Minimum: 144 A at power factor 1.0
Power factor: 0.80 Current: 180 A Reactive power: 75 kvar Poor

Assumes 100 kW, 400 V three-phase. Current = P ÷ (√3 × V × power factor).

Notice how the curve is steep at the left and flat at the right. Going from 0.7 to 0.8 saves a lot of current. Going from 0.95 to 1.0 saves very little. This is why engineers aim for about 0.95 and not for a perfect 1.0. Do not over-correct. Too much capacitance makes the current lead and can raise the voltage.

Where reactive power is managed

Reactive power matters most for voltage. As a simple rule, active power balance sets the grid frequency, and reactive power balance sets the voltage. When a lot of Q flows across a line, the voltage at the far end sags. When there is too much Q, the voltage rises. So the grid manages Q locally, close to where it is needed.

In factories, capacitor banks sit near large motors, as in our example. In substations, capacitor banks support voltage when load is high. On long, lightly loaded lines and cables, which make their own reactive power, shunt reactors absorb the extra. Generators help too. By changing their field current, they can supply or absorb Q. Synchronous condensers do the same without making any active power.

Modern grids also use power electronics. Devices such as the STATCOM change their reactive output within a few cycles. We cover them in the related guides below.

Key takeaways

  • Reactive power (Q) swings between source and load and does no net work. Its unit is the VAR.
  • It appears when current is out of step with voltage. Coils store energy in a magnetic field, so their current lags.
  • The powers form a right triangle: S² = P² + Q². The power factor is cos φ = P ÷ S.
  • Reactive power is not lost energy, but its current still heats wires and fills their capacity.
  • Inductive loads absorb Q. Capacitors supply it. Placing a capacitor near the motor cancels the swap locally.
  • Reactive power balance controls voltage. That is why the grid uses capacitors, reactors, generators and power electronics.
Learn more

Related guides on Electrical Blog:

How a STATCOM Works: A Simple Step-by-Step Guide

What Is a FACTS Device? Types, Working & Uses Explained

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