Summarize with AI
How does reactive power work?
Mis à jour le
March 4, 2026
6
Min reading
Reactive power often represents a subject that raises a lot of questions. Indeed, the latter may appear on business electricity bills.
Sans Electricity concept, it is difficult to grasp its usefulness and its role. That's good, because at SirEnergies we like to explain complex topics in a simple way!
So first we are going to explain to you what the nature of reactive power is. Afterwards, we will see its billing methods and therefore the cost that it can represent on your invoices. Finally, we will see how it is possible to avoid these billing penalties.
You may not be affected at this time. However, this does not presuppose the future. Indeed, it is likely that over time and wear and tear, some machines, for example industrial ones, will deteriorate. This may cause reactive power consumption to exceed an acceptable level.
The installation of certain machines, as we shall see, also promotes the consumption of reagent. For this reason, it is important to know this mechanism in order to prevent its risks. This problem particularly concerns industry professionals. The impact of individual customers is too low to appear significant for the network operator.
What is the difference between active power, reactive power, and apparent power?
To begin with, it is necessary to give the definition of electrical power. In the alternating regime, electricity is composed of three parts: apparent power, active power, and reactive power. So this does not apply to the DC regime.
Apparent power — Definition
Apparent power is the value that appears on your contract. It is the trigonometric sum of the other two powers. This value represents the threshold not to be exceeded in order to avoid disconnecting.
Active power — Definition
Active power is transformed into heat (Joule effect) or into movement (mechanical). It's this part that makes our lightbulbs sparkle. This is the useful power of electricity.
Reactive power — Definition
It is the “invisible” part of electricity. This reactive power is the consequence of the magnetic fields generated by certain devices in operation.
It should be understood that the more reactive power is consumed, the greater the volumes of energy transported will be. However, if the power volumes requested are large, this causes additional losses on the network. The cost of these losses is significant. Cable wear and heating lead to additional costs for the network operator.
Despite the fact that reactive power is useful for some machines, its cost is therefore passed on to your bill.
How to calculate reactive power?
In order to better understand this relationship, we can clearly state the following power formula:
S = √ (P² + Q²)
With respectively:
- S = Apparent power (VA → Volt × Ampere)
- P = Active power (W → Watt)
- Q = Reactive power (var → Volt × Reactive Ampere)
Representation in the form of vectors is also useful in understanding this phenomenon. This should probably bring back some high school memories for some of you!
The angle φ (pronounced “phi”) contained in the reactive power formula represents the phase shift of the electric wave. It is expressed in degrees. This figure is called the power triangle.
In summary
To simplify these explanations, we can summarize the situation like this.
The customer will pay for apparent power S while its devices will in reality only use active power P to function.
In fact, it is still necessary to maintain reactive power Q in order to maintain a balance between these elements. If reactive power consumption is too high, it is penalized.
Reactive power calculation — Power triangle
I won't cite the source of the drawing below, but those who know Emmanuel Sire will laugh 😂 It's a new version without a veil!
Below we have a person on the beach pulling a boat that is in the water using a rope.
Hard hard to pull a boat with a rope while being next to it...
We can make the analogy of reactive power with the forces at play in this diagram.
It is necessary for humans to exert vertical force to move the boat forward. However, we can see that another force comes into play, which is in turn horizontal. This force does not move the boat forward, but it is there. It's reactive power.
How is reactive power billed?
The companies most often affected by these problems use a certain type of installation. Among others, we can mention:
- Arc or induction furnaces
- Asynchronous motors
- Current transformers and power rectifiers
- Fluorescent lamps
The TURP plans to charge for reactive energy extracted during the months of November to March, from 6 a.m. to 10 p.m. But only for the part that exceeds the famous threshold of tangent φ (0.4).
First of all, it is necessary to calculate the following ratio TG (tangent φ): reactive energy/active energy.
If this ratio exceeds 40% (0.4), then the network operator charges for the excess. In fact, the excessive consumption of reactive energy therefore causes losses, as we have seen. However, it can also cause voltage drops or overloads (at the transformer level). It is therefore necessary to send a clear price signal to customers in order to encourage them to reduce their reactive power calls.
Reactive energy billing applies to customers connected to High Voltage HTA and HTB (C1-C2-C3) and Low Voltage BT > 36 kVA (C4).
Reactive energy is billed in this way:
The price is expressed in C€ per kvar·h.
The quantity of reactive energy and the price are shown on your bill. You can therefore calculate these amounts yourself using this table.
How do you avoid billing penalties?
It is possible to compensate for the reactive energy required for your operation by using fixed or automatically regulated capacitors.
It is therefore possible to supply reactive energy into your installation directly from theinterior of the installation and therefore without taking anything from the electrical network. In this way, the demand for reactive energy from the network is reduced. Penalties are thus avoided.
In this way, the site can become autonomous in terms of reactive power.
A capacitor bank installation on an industrial site. Source: ICAR
There are two different ways to produce reactive energy:
- A first constant: batteries deliver constant power over time. This concerns installations with low reactive consumption (less than 15% of the transformer power).
- A gradual second: the batteries deliver power that is activated in stages. Depending on the time chosen, it is possible to activate different levels of intensity.
This type of installation can be installed in various locations. First of all, upstream of the electrical site and thus cover the entire site. Or else be located locally on a specific machine. This flexibility makes it possible to adapt this system according to the needs of the site. This way you know more about reactive power and its impact on your electricity bill.
Do not hesitate to consult our article to understand everything about Electrical erasure.
Did you know that?
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