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Glossary · Energy industry & energy transition

Imbalance of a balancing group (Bilanzkreisabweichung)

Balancing group deviation Balancing group Balancing group responsibility Balancing energy schedule balancing energy electricity market

A balancing group imbalance is the amount of energy by which the quantities actually allocated to a balancing group deviate in a settlement interval from its binding final position. The final position results from the notified schedules and trades. The actual side consists of the measured or profile-based feed-ins and withdrawals that are allocated to this balancing group under the market rules. The European Electricity Balancing Guideline (Regulation (EU) 2017/2195, EB GL for short) describes the imbalance as a calculated energy volume: the difference between the volume allocated to a balance responsible party and its final position in an imbalance settlement period. In addition, there may be an "imbalance adjustment" that takes account of activated balancing energy. Depending on the sign, one speaks of an overcoverage (over-feeding) or an undercoverage (under-feeding) of the balancing group.

Quantity, unit and time grid

The imbalance is a quantity of energy and is usually stated in megawatt hours (MWh) or kilowatt hours per settlement interval. Under Article 53 EB GL, transmission system operators settle imbalances EU-wide in 15-minute periods. The boundaries of the market time units must coincide with those of the settlement intervals. In Germany, the quarter-hour was already the balancing grid beforehand. Under Section 4(2) of the Electricity Grid Access Ordinance (StromNZV), the balance responsible party is responsible for a balanced position between feed-ins and withdrawals in every quarter-hour.

An example shows the relationship between power and energy: a wind farm balancing group has planned and sold 100 megawatts of feed-in for one quarter-hour. In fact, the wind farm feeds in only 80 megawatts on average. The missing 20 megawatts over one quarter-hour (0.25 hours) result in an undercoverage of 5 MWh. It is this quantity of energy that is settled, not the instantaneous power difference.

Distinction from schedule, forecast error and balancing energy

The schedule indicates how much electrical power per unit of time is to be exchanged between balancing groups or fed in or withdrawn at feed-in and withdrawal points. It is thus the notified target position. The imbalance arises only when this position is compared retrospectively with the allocated actual quantities. A forecast error for wind, photovoltaics or consumption is a frequent cause, but not the same thing. If it is corrected in good time through trading or plant control, it does not lead to a settlement-relevant imbalance.

The term must also be distinguished from balancing energy. Transmission system operators deploy balancing energy to maintain the physical equilibrium and thus the frequency. The imbalance, by contrast, is a settlement and responsibility quantity. Physically, only the sum of all imbalances has an effect. Opposing imbalances of different balancing groups cancel each other out, so that an individual imbalance does not trigger any particular activation of balancing energy. In Germany, the net balance across the four control areas in the grid control cooperation (Netzregelverbund, NRV) is considered jointly.

Settlement via imbalance energy and reBAP

In terms of terminology, the distinction is often drawn imprecisely. Imbalance energy (Ausgleichsenergie) is the quantity of energy that is notionally delivered to or taken from a balancing group to compensate for its imbalance. Under the EB GL, the "imbalance settlement" is the financial mechanism by which these quantities are charged to or remunerated to the balance responsible parties. Each quarter-hour is valued at the imbalance price. This can be positive, negative or zero. In Germany, the cross-control-area uniform balancing energy price (reBAP) applies for this purpose. It is a symmetrical price which, in principle, is the same for over- and undercoverages in a quarter-hour.

The imbalance price is not a penalty for forecast errors. According to the German transmission system operators, it is the price of the balancing reserve actually activated. In a perfectly balanced system it would be zero. An imbalance can therefore also generate revenue, for example when a balancing group deviates in a "system-supporting" way, that is, against the overall balance of the grid control cooperation, or when balancing energy prices are negative. A party that, for example, over-feeds while the overall system is undercovered receives the then usually high reBAP for its surplus quantity. No lasting business model can be derived from this: balance responsible parties are obliged to manage their balancing groups in a balanced manner, and the outcome is uncertain in advance.

The price formation deliberately contains elements intended to make targeted exploitation of imbalance energy unattractive. Article 55 EB GL sets limits: for negative imbalances (undercoverage), the price must be at least equal to the weighted average price of the activated positive balancing energy from frequency restoration and replacement reserves. For positive imbalances (overcoverage), it may be at most equal to the corresponding price of negative balancing energy. In Germany, the Bundesnetzagentur has additionally introduced, in several determinations, a coupling to intraday exchange prices and a scarcity component for quarter-hours with a high NRV balance. Since December 2022, a calculation methodology based on decision BK6-21-192 has applied. The details are set out in the transmission system operators' current model description.

Why imbalances are relevant to the system

In the power system, generation and consumption must be balanced at every moment. The market distributes this task among the balance responsible parties and reflects it through responsibility and settlement, rather than planning all plants centrally (see balancing group responsibility). Under European law, market participants are to balance their positions themselves via the markets. The transmission system operators are to balance via the balancing reserve market only those imbalances that still remain after the end of the intraday market. Missing or surplus quantities are therefore to be procured or sold via the forward market, day-ahead and intraday trading, or one's own flexibility. Imbalance energy covers only the unavoidable remainder.

As long as imbalances offset one another, their physical effect remains small. But if many balancing groups deviate in the same direction, a common power error arises. This happens, for example, when solar feed-in was overestimated over a large area. The transmission system operators must then activate procured balancing capacity on a larger scale. Precisely in such situations, the scarcity component raises the costs for balancing groups that exacerbate the imbalance.

Typical causes and misinterpretations

  • Weather-dependent generation: wind and solar forecasts are getting better but remain uncertain and often change right up to shortly before delivery.

  • Consumption: industrial loads depend on production processes, household loads on temperature, time of day and behaviour.

  • Technology: power plant outages or power changes ramped more slowly than planned.

  • Data and processes: incorrect allocation of metering points, incorrectly notified schedules or corrections to measurement data. Balance responsible parties must check the data transmitted to them in good time and report objections without delay.

It is wrong to regard every imbalance as an individual failure. Even a professionally managed balancing group deviates because of unavoidable forecast uncertainty. Equally wrong is the assumption that imbalances are purely commercial bookings with no significance for grid operation. Their sum determines how much balancing energy is needed. Anyone who systematically uses imbalance energy as a cheap substitute for procurement on the market shifts risks onto the overall system. This is exactly what the price rules are meant to prevent.

Flexibility, storage and grid-serving behaviour

With a growing share of weather-dependent generation, an imbalance is often the remainder of a continuous correction process: new forecasts are continuously translated into schedules via intraday trading. Flexibility helps keep the remaining imbalance small. A storage unit can replace missing feed-in or absorb surpluses, and a flexible industrial process can shift load. These options are economically valuable because they avoid imbalance energy costs.

Balancing-wise compensation and grid-serving behaviour are not the same thing. A measure can balance the balancing group without relieving a local grid congestion. Conversely, a grid-side intervention can cause imbalances in the balancing group if it is not properly offset in balancing terms. Data quality, metering concepts, aggregation rules and clear responsibilities are therefore prerequisites for flexibility not generating new imbalances.

Responsibilities

For each balancing group, the grid users designate a balance responsible party to the competent transmission system operator. This party bears the financial responsibility for imbalances between feed-ins and withdrawals. Every feed-in and withdrawal point must be allocated to a balancing group. The transmission system operators carry out balancing group settlement and procure and activate balancing reserve. Distribution system operators and metering point operators supply the necessary quantity data. The reBAP of a settlement period is published retrospectively: according to the transmission system operators, quality-assured values appear from the eighth following working day and may be corrected until final publication on the 20th working day of the following month.

Sources

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Edited by eCMS on 11.11.2026 15:46

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