Emissions · Glossary

Definition and legal classification

Emissions are substances or effects that pass from a source into the environment. They include gases, particles, radiation, noise, vibrations, light and heat. The German Federal Immission Control Act (Bundes-Immissionsschutzgesetz, BImSchG) defines the term in relation to installations. Under section 3(3) BImSchG, emissions are the air pollution, noise, vibrations, light, heat, radiation and similar phenomena emanating from an installation. This legal definition applies to immission control law. In climate balances or corporate reports, the term is often used more broadly or more narrowly.

In the power system, the term usually means greenhouse gases and air pollutants. They are released when energy is generated, converted, stored, transported or used. For climate protection, carbon dioxide, methane, nitrous oxide and fluorinated gases matter most. In the power sector, three questions are decisive: How many emissions arise from supplying one kilowatt-hour? Where are they accounted for? And which installations, fuels or market rules cause them?

Emissions and immissions

Emissions arise at the source, for example at the stack of a power plant or through methane losses during the extraction and transport of natural gas. Immissions, by contrast, describe what acts on people, animals, plants, soil, water, the atmosphere or material assets at a given location, such as the fine particulate concentration in the air we breathe. Since the amendment of July 2024, section 3(2) BImSchG also expressly names the climate as a protected asset with regard to immissions. Greenhouse gases mix globally in the atmosphere. For them, it therefore hardly matters climatically where they are emitted. For sulphur dioxide, nitrogen oxides, fine particulates or mercury, by contrast, location is central. The local burden depends on where an installation is located, how it is operated and how its exhaust gases are cleaned.

Units: CO₂ equivalents, absolute and specific emissions

Greenhouse gas emissions are usually stated in tonnes of carbon dioxide equivalent (t CO₂-eq.). For this, the mass of each gas is multiplied by its global warming potential (GWP). A time horizon of 100 years (GWP100) is customary. National greenhouse gas inventories currently use the values from the IPCC's Fifth Assessment Report. According to these, one tonne of methane equals 28 tonnes of CO₂ equivalent and one tonne of nitrous oxide equals 265 tonnes. Per tonne, methane has a much stronger effect than CO₂ but stays in the atmosphere for a shorter time. Its conversion factor therefore depends heavily on the chosen time horizon: over 20 years it is markedly higher than over 100 years. CO₂ equivalents make different gases comparable but say nothing about the source. CO₂ from a coal-fired power plant, methane losses from a gas supply chain and nitrous oxide from a chemical plant can appear in the same unit, even though they have to be treated very differently in technical and political terms.

Absolute emissions state how many tonnes were released in total. Specific emissions relate this quantity to a unit of output. In the power sector, this is usually grams of CO₂ or grams of CO₂ equivalent per kilowatt-hour. This indicator, often called the emission factor, describes the emission intensity of a quantity of electricity. The two measures can develop differently. More wind and solar power can lower specific emissions while absolute emissions fall more slowly, because electricity consumption rises due to heat pumps, electric mobility and industrial electrification. Conversely, low consumption can mask a high emission intensity if the electricity comes predominantly from coal.

An example from Germany: According to calculations by the German Environment Agency (Umweltbundesamt, UBA), generating the electricity consumed domestically in 2025 caused on average 344 grams of CO₂ per kilowatt-hour. In 2024 it was 353 grams, and in 2023 it was 379 grams. The value for 2025 is provisional. It covers only the direct CO₂ emissions of generation, that is, neither other greenhouse gases nor upstream chains. As reasons for the decline, the UBA cites more renewable energy, lower consumption due to weak economic activity, and net electricity imports. The agency therefore points out that the falling factor says only limited things about lasting climate protection in the power sector. This shows how strongly an indicator depends on the system boundary and on the circumstances.

Emissions are not the same as energy consumption

A kilowatt-hour consumed does not automatically cause a fixed quantity of emissions. How much arises depends on the generation mix, and also on the time of consumption, on imports and exports, on the availability of power plants and on market rules. An additional load in an hour with plenty of wind and solar power has a different effect from the same load during a period of low wind and solar output (Dunkelflaute), when fossil power plants cover the residual load. Anyone who wants to assess the emission impact of flexible consumers, storage or electrolysers must therefore distinguish between average and marginal emission factors.

For an average emission factor, the emissions of an electricity mix are divided by the quantity of electricity generated or consumed in a period. It is suitable for statistics, long-term trends and simple balances. A marginal emission factor, by contrast, asks which power plant changes its output when one additional kilowatt-hour is consumed or saved. This view is important for short-term control, load shifting and flexibility. Both approaches are legitimate, but they answer different questions. An annual average does not explain what emissions are triggered by charging an electric car in a particular hour. Conversely, an hourly marginal analysis does not replace a national climate balance. Marginal values are, moreover, estimated with models, and the results differ markedly depending on the method.

System boundaries and accounting rules

National greenhouse gas inventories under the rules of the Framework Convention on Climate Change allocate emissions to the territory in which they arise. For electricity imports, the emissions therefore appear in the generating country, not in the consuming country. The UBA factor for domestic consumption, by contrast, takes the exchange balance into account, that is, the ratio of electricity imports and exports, and thus reflects a different perspective. Corporate balances usually distinguish between direct and indirect emissions. Direct emissions arise in a company's own installations. Indirect ones arise, for example, from purchased electricity, heat or intermediate products. This helps to assign responsibility. If the rule applied is unclear, however, double counting or gaps may result.

The word "emission-free" is particularly open to misunderstanding. Wind power and photovoltaic installations burn no fuel in operation and therefore cause no direct greenhouse gas emissions. Manufacturing, construction, maintenance and disposal, however, cause emissions in upstream and downstream processes. The same applies to nuclear power plants: ongoing operation releases hardly any CO₂. Uranium mining, fuel production, construction, decommissioning and final disposal, however, have their own environmental impacts. "Emission-free" is precise only if the boundary is stated, that is, direct operating emissions, life-cycle emissions or a sectoral balance.

Differences between technologies

In power generation, the largest differences lie between fuels and technologies. Coal-fired power plants, especially lignite-fired ones, emit a lot of CO₂ per kilowatt-hour. Coal is very carbon-rich, and electrical efficiency is limited. A gas-fired power plant releases markedly less CO₂ in combustion per kilowatt-hour generated, particularly as a combined-cycle plant. Methane losses during the extraction, transport and distribution of natural gas can, however, noticeably worsen the climate balance. How much depends on the supply route and on the global warming potential applied. Wind, photovoltaics and hydropower have very low life-cycle emissions. Their effect in the overall system, however, also depends on grid expansion, storage and flexibility, and on whether they actually displace fossil generation. Power plants with combined heat and power show a further accounting problem: how many emissions are attributed to electricity and how many to heat depends on the allocation method chosen.

Emissions trading: putting a price on emissions

The European emissions trading system (EU ETS 1) links emissions with costs. Operators of covered installations must surrender an emission allowance for every tonne of CO₂ equivalent they emitted in the reporting year. Covered are, among others, combustion installations with a total rated thermal input of 20 megawatts or more, that is, large power plants and combined heat and power plants, as well as certain industrial installations. Smaller combustion installations do not fall under the EU ETS 1. The system predominantly covers CO₂. Since 2013, the obligation has also applied to nitrous oxide in certain processes, for example in nitric acid and adipic acid production, and to perfluorocarbons from aluminium production. Methane losses in upstream gas supply chains are not covered by the emissions trading of power plants. Since 2024, installations for the incineration of municipal waste above 20 megawatts must also monitor and report their emissions. For the time being, they do not have to surrender allowances.

The resulting CO₂ price raises the variable costs of fossil power plants, and the more so, the more carbon-rich the fuel is. It thereby shifts the merit order of power plants and influences wholesale prices, power plant dispatch and investment. Emissions trading does not ban any technology. It limits the total quantity of allowances through a declining cap and makes emission rights scarce. How strongly it works depends on this cap, on the market stability reserve, on exceptions such as free allocation, and on investors' expectations.

Guarantees of origin and electricity labelling

Electricity labelling and green electricity tariffs, too, often cause misunderstandings. Guarantees of origin certify that a certain quantity of electricity was generated in renewable installations and was allocated, in accounting terms, to a consumption. They change neither the physical flow of electricity in the grid nor necessarily the short-term dispatch of power plants. For consumers, they make procurement decisions transparent. Whether emissions are actually lower depends on further questions: Are new renewable installations incentivised? Is fossil generation displaced? And is flexible demand shifted to times of low emission intensity?

What to look out for in emission figures

Source and scope: Does the figure apply only to CO₂ or to all greenhouse gases in CO₂ equivalents, and which global warming potential was used for the conversion?
System boundary: Are only direct emissions at the stack meant, or also upstream chains and the entire life cycle?
Reference: Is it about absolute tonnes or a specific figure per kilowatt-hour? And does the value refer to generation or to domestic consumption including the exchange balance?
Method and period: Is it an average value or a marginal value, an annual average or an hourly value, provisional or final data?
Legal framework: Is the term meant in the sense of immission control law, of emissions trading or of a voluntary corporate balance?

Emission figures for the power system contain several levels at once: conversion losses, fuel choice, market rules, international supply chains and national accounting boundaries. A single number is meaningful only if the source, unit, method and system boundary are stated. If this information is missing, a measured quantity quickly appears more clear-cut than it is, and technical causes, economic incentives and political responsibilities become confused.

Sources

§ 3 BImSchG – Begriffsbestimmungen — dejure.org (Gesetzestext Bundes-Immissionsschutzgesetz); unbekannt (Fassung ab 09.07.2024). § 3 Abs. 2 und Abs. 3 BImSchG
Treibhausgas-Emissionen — Umweltbundesamt; unbekannt. Abschnitt zum Global Warming Potential (GWP100)
CO₂-Emissionen pro Kilowattstunde Strom 2025 nur leicht gesunken — Umweltbundesamt; unbekannt (2026). Einleitung und Ursachenbeschreibung
TEHG-Anwendungsbereich — Deutsche Emissionshandelsstelle (DEHSt) im Umweltbundesamt; unbekannt. Abschnitte zur Verbrennungstätigkeit nach Anhang 1 TEHG
Wesentliche Änderungen des Anwendungsbereichs für stationäre Anlagen — Deutsche Emissionshandelsstelle (DEHSt); 2024-02-13. Newsletter, Abschnitt Abfallverbrennung

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Edited by eCMS on 11.11.2026 19:44