What is active and reactive power?

Overview meter readings and consumption

In electricity, a distinction is mainly made between two types of power in grids with an alternating current: active power and reactive power.

Active power

This is the power that is actually transformed into useful work. It is converted into heat, light or mechanical force. This is what directly makes your electrical appliances work (a bulb that lights up, a motor that spins, an oven that heats up, etc.).

It is expressed in kilowatt hours (kWh) on your electricity bill. The associated power is the Watt (W) or kilowatt (kW).

Reactive power

This is the power needed for some electrical equipment that uses magnetic fields to work, such as induction motors, transformers, or some industrial ovens.

It is not "consumed" directly to produce useful work, but it is essential to maintaining those magnetic fields. It flows without being transformed into work, but it does add a load onto the grid.

It is expressed in kilovolt-ampere reactive hours (kVArh). The associated power is the Volt-ampere reactive (VAr) or kilovolt-ampere reactive (kVAr).

A horse pulling a barge

Picture a horse on the bank pulling a barge along a canal. The horse is pulling the barge horizontally, along the canal. This is active power. It's this that makes the barge move, and is performing useful work.

But the horse can't always pull the barge perfectly straight along the canal. To keep the barge in line, or to compensate for the current or wind, the horse must exert a force at an angle. This force "alongside" the main movement doesn't contribute directly to the barge's progress, but it is essential for it to run properly. This is reactive power.

Am I charged for reactive power?

If you are a private individual with a traditional installation (LED lighting, household appliances, electric heating, etc.), you use virtually no reactive power, so it isn't measured by the smart meter.

However, for professionals and major consumers, reactive power may affect your bill if it exceeds a certain threshold. This is why these customers have a specific meter, the AMR meter, which can measure both active and reactive power.

Good to know

We only charge customers on a high-voltage rate for reactive power, and only if it exceeds the authorised threshold (cos phi of 0.9). Do you have an AMR meter and are connected to the low-voltage network? In this case, we'll only charge you for active power.

What to look out for when sizing power

When you measure the current absorbed using a clamp ammeter, it is the total apparent current that is indicated. The same applies to the value of a protection device: it will trip when the apparent current exceeds its rating. It is therefore always advisable to consider the total current when sizing and not just the active current.

In the event of an overcurrent trip, the solution is not always to reinforce the power. Reducing the reactive current can sometimes solve the problem. In some cases, installing capacitors can compensate for excess reactive energy. Don't hesitate to contact your electrician to determine the correct size of capacitors.

Our account manager will also be happy to answer any questions you may have.

Overview meter readings and consumption