Supply and Demand Balance

In the electricity system, the supply and demand balance refers to comparing, and continuously balancing, the electrical power provided and the power required. In balanced operation, generation, storage discharge and imports together equal consumption, including storage charging, exports and losses. Unlike an annual statistic, this balance describes an ongoing operational task: planned or unexpected deviations must be balanced out in good time.

A distinction must be made here between the desired equilibrium and the actual dynamics. Short-term imbalances occur in normal operation. They are first bridged, among other things, by changes in the kinetic energy stored in rotating machines, and are then eliminated by control and adjusted feed-in or withdrawals. The statement that supply and demand must match at all times is therefore an operational target condition, not a claim that deviations never occur.

Power, Energy and Balance Boundaries

Power is stated in watts, kilowatts, megawatts or gigawatts. It describes the energy provided or consumed per unit of time. Kilowatt hours and megawatt hours, by contrast, denote quantities of energy. A country may generate enough electricity on an annual average and still depend on additional power on a low-wind winter evening. A balanced annual quantity therefore does not prove that demand is met at every moment.

A simplified calculation example: if 100 megawatts are continuously missing for a quarter of an hour, 25 megawatt hours must be additionally provided, or a corresponding amount of consumption avoided. A battery with 100 megawatts of discharge power and 200 megawatt hours of usable energy content could, in calculation terms, deliver this power for two hours. This applies only if the state of charge is sufficient and if the figures already account for the usable output. High power alone therefore says nothing about how long it can bridge a shortfall.

Every balance needs a clearly defined boundary. In a national balance, cross-border physical electricity flows are recorded as imports and exports; across the entire interconnected area under consideration, they are internal flows. Likewise, own consumption and storage must not be counted twice: if the net feed-in of a power plant is already used, its deducted own consumption must not be counted again as additional withdrawal. Storage charging belongs on the demand side, storage discharge on the supply side.

What Grid Frequency Indicates

In the Continental European synchronous area, the nominal grid frequency is 50 hertz. A sudden power shortage initially causes the frequency to fall, and a power surplus causes it to rise. Synchronously coupled rotating machines release kinetic energy in the event of a shortage and slow down. How quickly the frequency changes depends, among other things, on the size of the imbalance and the inertia present.

However, frequency is not a direct meter of a German electricity deficit. It reflects the dynamics of the synchronously connected system; national borders are not frequency borders. Germany can import electricity and still be part of an overall balanced system. Moreover, the power currently missing cannot readily be derived from a single frequency value: the frequency trajectory, inertia and control already in effect are also relevant.

Distinction from Security of Supply and Grid Congestion

Security of supply is broader than the instantaneous balance. It includes sufficient available generation and storage capacity, a reliable fuel supply, functioning grids and safe operation. A local line fault can disconnect households from the grid even though enough generation is available overall. Conversely, a momentarily balanced position does not guarantee that a prolonged cold spell or a major plant outage can be managed.

Grid congestion must also be distinguished. Supply and demand may balance in total, while individual lines would be overloaded because of the spatial distribution of feed-in and withdrawals. Redispatch therefore changes the dispatch of plants at suitable locations: feed-in is reduced at one point and increased at another. The aim is to relieve the grid, not primarily to remedy a system-wide power shortage.

By way of illustration: a hypothetical area requires 60 gigawatts and has 60 gigawatts of feed-in. If a large part of generation is in the north but consumption is in the south, arithmetical coverage alone is not sufficient. In addition, the grid must enable the necessary transport. The power balance answers the question of quantity; the grid calculation checks spatial feasibility under technical security conditions.

How Planning and Ongoing Balancing Interact

Balancing is not achieved by a single control centre that directly controls every socket. In Germany, balance responsible parties bear responsibility for the balanced quarter-hour balance of their balancing group and for its schedule management. Besides assigned feed-in and withdrawals, the calculated balance also includes agreed deliveries between balancing groups. A balancing group is not a spatially closed electricity grid.

Planning begins in longer-term trading, is made more concrete for the following day and is adjusted in short-term trading. Forecast errors and disturbances nevertheless remain: wind and cloud cover develop differently than expected, consumers change their behaviour or power plants fail. Improved forecasts and short-term re-trading reduce the need for balancing, but do not replace physical control.

For remaining system deviations, the transmission system operators organise balancing energy. Here, balancing capacity (Regelleistung) refers to the power change held in reserve or callable, in megawatts; balancing energy as activated (Regelarbeit) is the energy quantity actually activated, in megawatt hours. Positive control increases feed-in or reduces withdrawal. Negative control reduces feed-in or increases withdrawal. A change in consumption can thus also stabilise the balance.

The types of balancing reserve fulfil different tasks. Primary control, also called FCR, first limits the frequency deviation. Secondary control, or aFRR, and minute reserve, or mFRR, support restoring the intended system balance. Their effects can overlap in time; they are not substitute power plants switched on in strict sequence.

Providers must demonstrate the respective technical and organisational requirements through prequalification. Decisive factors include, for example, responsiveness, reliability, metering and controllability. Suitable power plants, storage units and flexible consumers can contribute individually or in pools. However, a small unit is not automatically capable of providing balancing reserve simply because it can change its output.

The quarter-hour balance and continuous physics are different levels here. A balancing group can be balanced in calculation terms over 15 minutes and still temporarily draw too much or too little power within that period. For grid operation, the speed and simultaneity of power changes therefore also count, not only the energy sum at the end of the settlement interval.

Renewable Energies and Flexible Demand

As the shares of wind and solar power grow, a larger part of supply follows the weather. At the same time, heat pumps, electric vehicles and electrolysers change demand. Flexibility thus gains importance: generation, storage and consumption must be able to adapt to changing conditions within their technical and economic limits. However, more renewable generation does not automatically mean a proportionally rising need for balancing reserve; forecast quality, trading and control cooperation also have an effect.

An important analytical variable is the residual load. It is often understood as demand minus the simultaneous feed-in from wind and solar; the precise definition must be stated. At high residual load, other generators, storage discharge, imports or a reduction in demand are needed. At negative residual load, the weather-dependent generation considered already exceeds the underlying demand. Storage charging, additional flexible demand, exports or a reduction in feed-in then come into question, among other options.

"Surplus electricity" does not remain in the grid as an unused quantity of electricity. It usually means a temporally and spatially limited oversupply under particular market or grid conditions. Either feed-in and withdrawals are adjusted or an imbalance arises. A negative exchange price is a market signal in this context, but in itself neither proof of line overload nor of an impermissible frequency.

Demand flexibility is also limited. An electric vehicle must be sufficiently charged by departure, a refrigeration system must keep within its temperature limits and an industrial process must meet its production requirements. Shifted consumption may have to be made up later. For the flexibility that is actually usable, power, possible duration, lead time and knock-on effects therefore count, as do metering systems, tariffs, automation and contractual responsibilities.

Availability, Imports and Typical Misinterpretations

Installed capacity is not the same as available capacity. Photovoltaic systems deliver no solar power at night; an installed power plant can be unavailable because of maintenance, a fault or lack of fuel. For storage, state of charge and energy content limit the duration of use. For meeting demand, actual availability, response speed, possible operating duration and reachability in grid terms therefore count.

Imports are also not an unlimited reserve. They can support the balance if exportable power and sufficient transmission capacity are available abroad at the same time. Large-scale weather situations can affect several countries at once. Conversely, exports do not prove a lasting domestic surplus: trade direction and electricity flows can change with prices, generation conditions and grid restrictions. For the analysis, the specific time and physical transferability are decisive.

Costs, Incentives and Responsibilities

In Germany, balance responsible parties bear financial responsibility for remaining balance deviations. These are settled per quarter-hour at the uniform cross-control-area balancing energy price, abbreviated reBAP. Depending on the sign of the deviation and of the price, a payment or a credit may result. Balancing energy settlement should therefore not be equated with a flat-rate penalty; the obligation to manage the balancing group properly remains.

The holding of balancing capacity and the call of activated balancing energy must be distinguished economically. Holding is financed through transmission network charges, while activation is settled via the balancing energy system. Balancing energy serves system security and is not a freely available substitute instrument for procurement omitted by a balance responsible party.

In addition, costs arise for grid congestion management, storage, flexible demand and metering and control technology. These items must not be added up indiscriminately as costs of balancing energy. Low generation costs per kilowatt hour do not yet answer what a supply that is usable at all times costs. The time of provision, location and available balancing options are also decisive here.

The practical value of the supply and demand balance thus lies in three distinctions: energy quantity is not instantaneous power, arithmetical coverage is not assured transport, and technical flexibility is not automatically reliably available market capacity. Only together do these perspectives enable a robust assessment of the electricity system.

Sources

Regelreserveprozesse 2030 – Das Grünbuch der deutschen Übertragungsnetzbetreiber für die Regelreserve — 50Hertz, Amprion, TenneT und TransnetBW; 2024-12-16. Kapitel 2 und 2.1, Seiten 5–6; Kapitel 3, insbesondere Seiten 9–11; Abschnitt 5.2.2
Project Inertia – Phase II: Updated Frequency Stability Analysis in Long Term Scenarios, Relevant Solutions and Mitigation Measures — ENTSO-E; 2023-11-08. Abschnitt 3.1, Seite 12; Abschnitt 3.2, Seite 13
Bilanzkreisführung — Amprion GmbH; unbekannt. Einleitungsabsatz und Abschnitt „Ermittlung und Abrechnung von Bilanzkreisabweichungen“
Wie funktioniert der Regelreservemarkt? — Deutsche Übertragungsnetzbetreiber, gemeinsame Plattform regelleistung.net; unbekannt. Schritte 1–4: Präqualifikation, Ausschreibungen, Aktivierung und Abrechnung
Redispatch — Bundesnetzagentur; unbekannt. Begriffsdefinition am Seitenanfang

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Edited by eCMS on 11.11.2026 13:50