Auxiliary power consumption of a power plant · Glossary

Definition and delimitation

The auxiliary power consumption of a power plant is the electrical energy that a power plant needs for its own operation. In energy statistics this quantity is often called power plant own consumption (Kraftwerkseigenverbrauch). It refers to the electrical work of a power plant's ancillary and auxiliary systems. According to a widely used statistical definition, it also includes the losses of the machine transformers, i.e. the unit transformers between the generator and the grid. Auxiliary power consumption includes, among other things, feedwater pumps, cooling water pumps, forced-draught and induced-draught fans, conveyor belts, fuel preparation, flue gas cleaning, water treatment, control systems, converters, lighting in the plant area, compressed air supply and other auxiliary units.

The central equation is: net electricity generation equals gross electricity generation minus auxiliary power consumption. Gross electricity generation is measured at the generator terminals. Net electricity generation is the quantity that can be delivered externally after deduction of auxiliary power consumption. Official statistics in Germany describe own consumption as the energy consumption that sustains the plant's production process. Electricity that the plant obtains from third parties is not counted.

Units and worked example

As a quantity of energy, auxiliary power consumption is stated in kilowatt hours, megawatt hours or gigawatt hours. For ongoing operation, the auxiliary power demand also matters. This is the electrical power, in kilowatts or megawatts, that the auxiliary systems draw at a given moment. Auxiliary power consumption is often expressed as a share of the gross figure. An example: a power plant generates 1,000 megawatt hours gross and consumes 60 megawatt hours of it itself. Net electricity generation is then 940 megawatt hours, and the auxiliary share is six percent. If the 60 megawatt hours are related to the net quantity, the result is about 6.4 percent. For percentage figures it must therefore always be clear what they refer to.

The same distinction applies to output; see gross output and net output. Gross output is measured at the generator terminals. Net output is gross output minus the electrical auxiliary power demand, i.e. what is delivered to the supply system. Depending on technology and operating state, a unit with 1,000 megawatts of gross output may, for example, deliver only 930 to 950 megawatts net. These are example values, not standard values. Anyone who adds up gross outputs in statistics, security-of-supply calculations or capacity assessments without deducting auxiliary power consumption overestimates the output actually available.

What is not part of auxiliary power consumption

Auxiliary power consumption is not the same as grid losses. Grid losses arise during transmission and distribution via the lines, transformers and switchgear of the public grid. Auxiliary power consumption, by contrast, arises within the generating plant and in its directly associated ancillary installations. By statistical convention, the boundary usually lies downstream of the machine transformer: its losses count as power plant own consumption, whereas losses in the downstream grid do not. Some popular accounts also include transmission losses in net generation. This does not correspond to the official definition. General electricity consumption at the site, for example for administrative buildings, canteens or workshops, likewise does not count as auxiliary power consumption, insofar as they are not part of the electricity generating plant. Where exactly the boundary lies depends in practice on metering concepts, statistical rules and contracts.

The term own consumption can also be misleading. For photovoltaic systems on buildings it usually means the electricity that the operator uses itself instead of feeding it into the grid. For a power plant's auxiliary power consumption, on the other hand, it concerns auxiliary energy without which the plant cannot generate, regulate, cool, clean or safely shut down. If an industrial company operates its own power plant and uses its electricity for its production, this is not power plant auxiliary consumption. This electricity is already net generation and is consumed outside the electricity generating plant.

Pumped-storage power plants are a special case. The electricity with which the pumps lift water into the upper reservoir is not auxiliary power in the narrower sense, but energy that is being stored. Energy statistics therefore usually report it separately. Electricity tax law, by contrast, expressly treats it as electricity for electricity generation.

Technical influencing factors

How high auxiliary power consumption is depends strongly on the technology. Steam power plants need a lot of auxiliary energy for the water-steam cycle, the feedwater pumps, cooling, fuel logistics and flue gas treatment. In coal-fired power plants, coal mills, conveying systems, dust removal, desulphurisation and denitrification are added. As a rough order of magnitude, a few percent of gross output applies to coal-fired power plants. Nuclear power plants need electricity for cooling circuits, pumps, safety systems and control systems, even after shutdown, because decay heat must be removed. A gas-fired power plant usually has a lower share. However, it needs auxiliary systems for fuel supply and, where applicable, gas compression, lubrication, cooling and control. Wind and photovoltaic installations have comparatively low auxiliary consumption, for example for control, tracking, heating, cooling or communication. But auxiliary consumption does not disappear entirely there either.

The share is also not a fixed property of a plant. Many auxiliary units keep running at part load although the quantity of electricity generated falls. In part-load operation, the auxiliary share therefore often rises. During start-up, auxiliary systems need electricity before the generator itself delivers. This energy then comes from the grid, from other units at the site, from batteries or from diesel generators. Auxiliary consumption can also occur during standstill, for example for frost protection, monitoring, lubrication, cooling or safety systems. Electricity that an idle power plant draws from the grid is often treated differently in statistics than auxiliary consumption covered from its own generation.

Efficiency, emissions and economic viability

For net efficiency, auxiliary power consumption has a direct effect. A power plant may reach a certain gross efficiency at the generator, but because of auxiliary consumption it delivers less usable electrical energy externally. For fuel consumption, specific emissions per kilowatt hour and economic viability, net generation is therefore usually the appropriate reference quantity. If auxiliary consumption is disregarded, plants appear more efficient than they actually are for the power system. In addition, the electricity consumed internally cannot be sold, but it causes fuel use and wear.

A high auxiliary consumption is nevertheless not automatically a sign of poor technology. Flue gas desulphurisation or denitrification requires electricity and thus increases auxiliary consumption. At the same time, it reduces pollutant emissions and fulfils environmental law obligations. Anyone comparing auxiliary shares should therefore know which functions the auxiliary systems perform.

Security of supply and black start

A power plant is not operational merely because of its fuel stock and generator output. It needs functioning auxiliary systems, a secured power supply for these systems and clear procedures for disruptions. If the grid fails, many power plants cannot restart without an external power supply. Plants capable of a black start are therefore specially equipped: they cover their auxiliary power demand themselves without the interconnected grid and can subsequently supply parts of the grid or further power plants. In scenarios on security of supply too, it is decisive whether gross or net values are used. This determines whether sufficient output is available on paper or whether part of it is already tied up in the plant itself.

Metering, statistics and law (Germany)

At the grid connection point, net feed-in is generally what is measured. To determine gross generation and auxiliary power consumption, additional measurements in the power plant are necessary. In the electricity market and for allocation to a balancing group, the net quantities fed in count. German energy statistics record gross and net electricity generation separately. Publications on a gross basis and on a net basis therefore differ significantly.

German electricity tax law also links to auxiliary power consumption. Electricity drawn for electricity generation is exempt from tax under section 9(1) no. 2 of the Electricity Tax Act (Stromsteuergesetz). Section 12 of the Electricity Tax Implementing Ordinance (Stromsteuer-Durchführungsverordnung; version following the amendment as of 1 January 2025) specifies this. Electricity is privileged that the ancillary and auxiliary installations of an electricity generating unit consume to generate electricity in the technical sense, for example for water treatment, steam generator feedwater supply, fresh air supply, fuel supply or flue gas cleaning. The list is not exhaustive. The customs administration requires that the electricity contributes directly to the technological process of electricity generation and is indispensable for it. What belongs to it in an individual case is repeatedly the subject of court decisions. If there are no metering devices with which tax-exempt and taxable quantities can be separated, and the electricity is obtained as a final consumer, the privilege is available only as a subsequent tax relief under section 12a. The tax-law and the statistical concept need not coincide completely in this respect.

Assessment

Auxiliary power consumption is a precise technical correction, but not an overall assessment of a power plant. Whether a plant is system-supporting, economical or climate-compatible additionally depends on fuel, efficiency, emissions, flexibility, availability, start-up behaviour, minimum load, grid connection and market rules. The term above all shows that generation is not the same as usable delivery. Anyone comparing outputs, efficiencies or electricity quantities should therefore state whether gross or net figures are used and which consumption within the plant has been deducted.

Sources

Begriffsdefinitionen zur Elektrizitätsstatistik (Studienbeschreibung, Datenbestand 70152) — GESIS – Leibniz-Institut für Sozialwissenschaften (Datenbestandskatalog); unbekannt. Definitionen Bruttoleistung, Nettoleistung, Bruttoerzeugung, Nettoerzeugung, Kraftwerkseigenverbrauch
Produzierendes Gewerbe: Stromerzeugungsanlagen der Betriebe im Verarbeitenden Gewerbe sowie im Bergbau und in der Gewinnung von Steinen und Erden — Statistisches Bundesamt (Destatis); unbekannt. Erläuterungen/Begriffsdefinitionen Bruttostromerzeugung, Nettostromerzeugung, Eigenverbrauch
Glossar: Bruttostromerzeugung — Umweltbundesamt; unbekannt. Glossareintrag
§ 12 StromStV – Strom zur Stromerzeugung — lxgesetze.de (Wiedergabe der Stromsteuer-Durchführungsverordnung); unbekannt (Fassung nach Änderung zum 01.01.2025). § 12 Abs. 1 und Abs. 3 StromStV
Steuerentlastung nach § 12a StromStV — Generalzolldirektion (Zoll online); unbekannt. Abschnitt Begünstigte Entnahmezwecke

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Edited by eCMS on 11.11.2026 19:42