How is reactive power transmitted via transformers

Reactive power in the power grid: How to avoid unnecessary energy costs

Reactive power not only burdens the power grid, it also represents an additional cost factor for companies. If the unusable reactive power in the grid exceeds a certain limit, it is billed to the electricity customer. In the following article we explain what reactive power is, how it is generated and how consumers can reduce or even avoid reactive energy costs.

How is reactive power created?

Reactive power is actually a misleading term. It describes the portion of the electricity in the power grid that is not converted into usable energy. This part of the electricity cannot be used to operate systems or devices - hence its name.

However, the reactive power is not entirely superfluous. It is necessary, for example, to build up and reduce magnetic fields in transformers, generators, electric motors or ballasts or to charge capacitors. The magnetic fields ensure that the electricity is transmitted in the alternating current network. Without reactive power, usable electricity would not come from the power plant to the consumer.

The proportion of reactive power increases especially when the electricity is fed into the grid irregularly. This is the case with electricity, for example, which is obtained from the renewable energies wind and sun. If too much electricity is fed into the grid, additional reactive power must be transferred to regulate the resulting voltages.

Definition: reactive power, active power, apparent power

  • Reactive power builds magnetic fields and is required to transfer the real power. It flows back and forth in the power grid between producer and consumer, but cannot be converted into other forms of energy.
  • Real power is the energy that is actually converted into other forms of energy and that the consumer can use as kinetic energy, heat or light.
  • The Apparent power is made up of the active power and the reactive power. It describes the total amount of electricity made available in the power grid, of which a certain proportion - reactive power - cannot be used.

A popular example illustrates the connection using a glass well filled with beer and head of foam. The beer corresponds to the active power, the foam to the reactive power. Both together result in the apparent power, because the glass could have been filled further (and more effectively) without the foam.

What does the reactive power cost the consumer?

What exactly is going on in the power grid is not only interesting for physics teachers and beer lovers. There is a simple reason for this: if a certain limit is exceeded, consumers have to pay for the reactive power contained in the apparent power. Because the energy suppliers have to transmit the apparent power so that the consumers can use the real power. The agreed performance factor is recorded in the collective bargaining agreement of the utility company and must be observed by the consumer.

This affects companies whose systems generate correspondingly large electromagnetic fields. You can find this cost factor as a separate item on the electricity bill. Private customers, on the other hand, are not burdened with the costs caused by reactive power.

Reduce reactive power costs

The reactive energy costs can be reduced or even avoided completely. A so-called reactive current compensation system is required for this. These systems work as follows: They generate the necessary reactive power and take it up again. This reduces the proportion of reactive power drawn from the energy supplier or even relieves the load on the power grid entirely. Accordingly, the utility bills the electricity customer for less or no reactive power at all.

A system for compensating reactive current can be retrofitted at the consumer without major intervention. The investment costs pay for themselves on average in two to three years. In individual cases, amortization times of less than a year are also possible. The system works most effectively when it is installed promptly and in a consumer-friendly manner. This is the most effective way of reducing the reactive current oscillating in the power grid.

The profitability analysis should be carried out by a competent expert. The suitable system should also be selected together with him. And with the correct maintenance of the reactive current compensation system, you ensure more performance and lower costs.

Advantage: reactive power compensation also lowers investment costs

The acquisition of a compensation system not only helps to save electricity costs. It can also help avoid the cost of an upgrade investment. This is the case, for example, when the output of the company's own transformer is not sufficient for a planned expansion of the machine park and a new, larger transformer would therefore have to be purchased. However, such a system may reduce the reactive power to such an extent that the output of the old transformer is sufficient for the new consumer devices. Then the company can do without investing in a new transformer.

The reason: When the reactive power drops, the power grid is relieved. More capacities are then available in the network. In the ideal case, the apparent power now corresponds to the real power in the relieved power grid. When calculating the profitability of the investment, the electricity customer compares the costs for a new transformer with the costs for a system for reactive power compensation and the possible reduction in electricity costs.

Read more about the advantages of reactive power compensation, also known as reactive current compensation, in my article "It pays to compensate for reactive current".

Lowering the CO2Emissions

In addition, the compensation of reactive power helps reduce CO2Emissions. The Central Association of the Electrical and Electronics Industry (ZVEI) assumes that there is potential in Germany to reduce grid losses in this way by 1.7 billion kWh (1.7 TWh / a) annually. This corresponds to the electricity consumption of 480,000 households and an emission of 950,000 tons of CO2, according to the ZVEI. The glass of beer can then certainly continue to be enjoyed with foam with a clear conscience.


Reactive power cannot be converted into usable energy and is therefore not available for the operation of systems. But it is not superfluous: it is necessary to transfer the usable electricity (the active power) in the AC grid from the power plant to the consumer. However, this puts additional strain on the strained power grids. If the contractual agreements with the utility company are not complied with, the energy suppliers will invoice their companies for the reactive power. A reactive current compensation system can help. The installation of such a system offers several advantages: It reduces the proportion of reactive current in the power grid and thus lowers electricity costs. In addition, better use of network capacity can avoid investment costs and - last but not least - reduce CO2- contribute to emissions.