Reactive power refers to the proportion of electrical power in an AC network that results from a phase shift between current and voltage. In contrast to active power, reactive power does not do direct work, but is used to build up and maintain electrical and magnetic fields in devices such as electric motors, transformers or capacitors. Although reactive power is essential for the operation of many electrical systems, it represents an additional load on the grid: It causes a flow of electricity that does not transmit any active power and thus demands lines and equipment. In grid operation, reactive power is therefore considered to reduce efficiency and should be kept as low as possible.
Reactive power is clearly distinguishable from power loss. While power loss is usually caused by ohmic resistors and is emitted in the form of heat, reactive power is a necessary side effect of AC operation and required for the construction and maintenance of fields.
Example: An electric motor with an apparent power of 1000 VA and a power factor (cos φ) of 0.9 can only provide 900 W of active power. The difference — the reactive power — is caused by the interaction of voltage and current in the motor. Reactive power “oscillates” back and forth between motor and network, meaning it is not lost, but oscillates back and forth. In order for 900 W of active power to be provided, 1000 VA of apparent power must be transmitted; the networks must therefore be designed for this apparent power.
A detailed technical explanation of the origin of reactive power can be found at the end of this glossary entry.
Since reactive power loads the power grid without being immediately usable, its share is kept as low as possible, both technically and economically. However, with the decline in conventional power plants and the expansion of renewable energy sources, the need for targeted, flexible provision of reactive power is significantly increasing. This is necessary to ensure voltage and grid stability.
Network operators are responsible for ensuring the required reactive power. Up to a certain extent, the provision of large battery storage is regulated in the respective Technical Connection Requirements (TAB) and the Technical Connection Guidelines (TAR) published by the VDE.
Depending on the network operator, the required proportion of reactive power is often around 10% of the connected power. For example, a large battery storage system that feeds in 10 MW of active power must provide an additional 1 MVar of reactive power if a static reactive power supply of 10% is required.
Plant operators do not receive any separate remuneration for this mandatory provision, although this entails additional costs. Since 2025, however, it has been possible to provide and pay for reactive power in addition to the minimum requirements as part of a market-based process.
Large-scale battery storage systems offer a wide range of system services in the power grid, including the targeted provision of reactive power — regardless of whether energy is stored or stored out. They can therefore react quickly and flexibly to the network operator's needs.
According to technical connection guidelines, battery storage systems are generally required to provide a defined proportion of reactive power. In addition, since the introduction of the reactive power market in 2025, additional, market-based provision of reactive power has been possible. The first tenders took place in summer 2025 and will be carried out regularly in the future. With this entry into the market, distribution system operators are now also more closely involved in procurement, which means that large battery storage systems are becoming more important at all voltage levels.
A simplified example serves as an illustration: An electric motor consumes an apparent power of 1000 VA. During operation, the motor builds up electric and magnetic fields. If the motor is operated with 50 Hz AC voltage — as is usual in Europe — there are 50 maximums of voltage and amperage per second, every 20 milliseconds. The inductive effect of the motor delays the current curve compared to the voltage, for example by 1.4 milliseconds. This corresponds to a phase shift of approximately 26° (phi = 26°, cos φ = 0.9).
Power is the product of voltage and current. As a result of the phase shift, negative power is temporarily generated when voltage and current have opposite signs. Part of the positive power is thus cancelled out — that is the reactive power. In the example, the effective power used by the motor is reduced by the factor cos φ = 0.9. Of 1000 VA apparent power, only 900 W active power is available. The inductive reactive power is sin (26°) × 1000 VA, i.e. approx. 440 Var (“volt-ampere reactive”). The power grid must be designed for the entire apparent power in order to be able to provide the desired active power.
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