CO₂ intensity · Glossary

CO₂ intensity refers to the amount of carbon dioxide that arises per unit of electricity generated or consumed. In the power system it is usually expressed in grams of CO₂ per kilowatt hour, i.e. g CO₂/kWh. Figures in kg CO₂/MWh and in t CO₂/GWh are equivalent, since 1 g/kWh corresponds to 1 kg/MWh. The measure is specific. It does not state how much is emitted in total, but how emission-intensive one kilowatt hour is within a defined system boundary. Outside the electricity sector, the term is also used for other reference quantities, for example CO₂ per unit of economic output. This entry deals with the electricity meaning.

The system boundary is central to the concept. A CO₂ intensity can refer to a single power plant, a national electricity mix, a company's consumption, one hour in a grid area or a contractually procured quantity of electricity. Without this information the figure is incomplete. A lignite-fired power plant has a different CO₂ intensity than a gas-fired plant, and a wind farm a different one than a country's average. The annual average of a power system can also deviate considerably from the values of individual hours.

The kilowatt hour is a quantity of energy. It must not be confused with power, which is measured in kilowatts or megawatts and describes instantaneous consumption or generation. CO₂ intensity per kilowatt hour answers the question of how emission-intensive a quantity of electricity is. It leaves open when this quantity arises, which power plants are ramped up or down for it, and what requirements for the grid and flexibility are associated with it.

A simple calculation example shows the application: emissions = quantity of electricity × CO₂ intensity. A household with annual consumption of 2,500 kWh would, at an average of 344 g CO₂/kWh, arithmetically come to about 860 kg CO₂. However, the result applies only to the chosen system boundary. Here these are direct CO₂ emissions in the German annual average, without upstream chains and without taking into account when the electricity was consumed.

Emission factor, electricity mix and accounting boundary

The closely related term emission factor also describes emissions per unit, for example per fuel, process or quantity of electricity. For electricity, both terms are often used synonymously. The German Environment Agency (Umweltbundesamt, UBA) expressly also calls its indicator for direct CO₂ emissions per kilowatt hour the emission factor or specific emission. The distinction by reference quantity is more precise: the emission factor of a power plant describes its own generation. The CO₂ intensity of an electricity mix describes the average of a mixed quantity of generation or consumption.

With the electricity mix, it must be clarified whether the generation mix or the consumption mix is meant. The generation mix covers the quantities of electricity generated within an area. The consumption mix additionally takes into account imports, exports and calculatory allocations. With heavy electricity exchange, the difference can be considerable. For Germany, the UBA therefore reports several variants, with and without taking the electricity trade balance into account. According to its analysis, the difference between the two accounting methods in 2024 was around 19 g CO₂/kWh. Two national average values for the same year can therefore differ depending on the method.

It is also important whether only CO₂ or CO₂ equivalents are meant. CO₂ equivalents, abbreviated CO₂e, combine, besides carbon dioxide, also methane, nitrous oxide and other greenhouse gases into a common measure of climate impact. In fossil electricity generation, CO₂ from combustion usually dominates. However, upstream emissions, for example from the extraction and transport of natural gas, can noticeably change the assessment. A figure in g CO₂/kWh is therefore not the same as one in g CO₂e/kWh.

A further boundary separates direct emissions from life-cycle emissions. Direct emissions arise during operation, for example when burning coal or gas. Life-cycle emissions additionally include construction, fuel extraction, transport, maintenance and decommissioning. In its Fifth Assessment Report, the Intergovernmental Panel on Climate Change (IPCC) compared both measures for various generation technologies. According to the median values compiled there, electricity from coal causes around 820 g CO₂e/kWh over the life cycle, from combined-cycle gas plants about 490 g, from photovoltaics around 41 to 48 g, from nuclear energy about 12 g and from wind energy 11 to 12 g. The direct emissions of coal and gas plants are lower, with medians of about 760 and 370 g CO₂e/kWh respectively. Wind and solar power cause practically no direct emissions in operation, but over the life cycle they are not zero. These values come from a literature review as of 2014 and show orders of magnitude. They are not current measured values of individual plants.

For many questions of the electricity market, direct emissions are considered because in the short term they depend on power plant dispatch. For complete climate balances, the life-cycle perspective is more informative. At the UBA the difference is clearly visible: for 2024, the direct CO₂ factor according to preliminary data is 353 g/kWh; including upstream emissions, the result is 414 g CO₂e/kWh.

Order of magnitude using Germany as an example

According to the German Environment Agency, an average of around 344 g CO₂ per kilowatt hour of electricity consumed in Germany was emitted during generation in 2025. For 2024 the UBA states 353 g, for 2023 379 g. Around 1990 the value was about 765 g CO₂/kWh. The value for 2025 is an estimate; the one for 2024 is based on preliminary data.

The figures are revised retrospectively. In an earlier publication, the UBA had still stated 363 g for 2024 and 386 g CO₂/kWh for 2023. Anyone using such values should therefore state the publication status and method. As causes of the decline, the UBA at that time cited a higher share of renewable energies, reduced electricity consumption due to economic stagnation, and the import surplus. The last two factors in particular show that a falling average value says only limited things about lasting climate protection measures in the electricity sector.

Average and marginal CO₂ intensity

Average and marginal CO₂ intensity are often confused. For average CO₂ intensity, the emissions of a power system are divided by the quantity of electricity generated or consumed in a period. It is suited to year-on-year comparisons, climate reports and the description of long-term decarbonisation.

Marginal CO₂ intensity, also called the marginal emission factor, describes the emissions that one additional kilowatt hour of consumption triggers or one saved kilowatt hour avoids. It depends on which power plant at the margin of the system responds to a change in demand. In many hours this is a fossil-fuelled power plant. In other hours, additional demand can absorb renewable surpluses or reduce curtailment. The marginal perspective is therefore particularly important for flexibility, load shifting, electrolysers, heat pumps, battery storage and electromobility. However, marginal values cannot be measured directly. They are estimated using models or statistical methods and turn out differently depending on the approach.

An annual average can obscure this effect. If an electric car is assessed using the average annual electricity mix, only a rough climate balance results. For controlling the charging process, hourly CO₂ intensity is more informative: charging when there is a lot of wind and solar power has a different effect than charging in hours when additional demand mainly draws on fossil generation. This is no longer only about the quantity of electricity, but also about when it is consumed.

Why CO₂ intensity is relevant in the power system

CO₂ intensity shows how far electricity generation is actually decarbonised. Installed renewable capacity alone is not sufficient for this. A power system can have many wind and solar plants and still show a high CO₂ intensity in certain hours if fossil power plants cover the residual load. Conversely, a high share of renewable generation can greatly reduce the CO₂ intensity of individual hours. However, questions of security of supply, grid congestion and storage needs are thereby not yet resolved.

For electrification the concept is particularly important. Heat pumps, electric cars and electric industrial processes increase electricity consumption, but can reduce the use of oil, gas or coal in other sectors. Rising electricity demand is therefore not automatically negative in terms of climate policy. What matters is which fossil fuels are replaced, how efficient the electric application is and with what CO₂ intensity the additional electricity is supplied. A heat pump can avoid considerable emissions even with an electricity mix that is not yet fully renewable, because it provides several kilowatt hours of heat from one kilowatt hour of electricity.

CO₂ intensity also plays a role for market rules and investment incentives. The EU emissions trading covers, among other things, the combustion of fuels in installations with a total rated thermal input of more than 20 MW. Installations for the incineration of hazardous waste are excluded. Municipal waste incineration installations have been included since 2024 only for monitoring and reporting. For covered fossil power plants, the CO₂ price makes generation more expensive depending on the emission factor. Coal power plants are thereby burdened more heavily than gas power plants. Wind, solar and hydropower plants incur no such costs in operation. For biomass, separate accounting rules apply, which are tied to sustainability criteria. Smaller combustion installations are not covered by EU emissions trading, but may be subject to national pricing systems. The CO₂ price influences the merit order of power plants and investments in low-emission generation, storage and flexible demand. It does not replace grid planning and rules on security of supply.

Typical misinterpretations

A low average CO₂ intensity does not mean that every kilowatt hour consumed is low in emissions. Physically, electricity is supplied at every moment by the plants currently feeding in and by imports. Guarantees of origin, certificates or green electricity contracts change the accounting allocation, but not automatically the generation in a particular hour. For corporate balances, contractual electricity procurement can be important. For the actual effect on the system, what additionally matters is whether new renewable generation is created, whether it matches consumption in time, and how the remaining load is covered.

It is equally problematic to equate CO₂ intensity with absolute emissions. A power system can lower its CO₂ intensity and still cause high total emissions if the quantity of electricity grows strongly or fossil generation remains in the system for a long time. Conversely, more electricity can go hand in hand with falling total emissions if it replaces fossil energy in transport, heating or industry. The kilowatt hour of electricity is then only one part of the balance, since the emissions avoided outside the electricity sector belong to the assessment.

It is also a simplification to use CO₂ intensity as a quality attribute of individual consumers without considering their load profile. A data centre, an electrolyser or a charging fleet with the same annual consumption can have very different effects. This depends on whether they run constantly, respond to price signals or deliberately use hours with low CO₂ intensity. The indicator must therefore always be read together with time resolution, controllability and grid integration.

Regional differences are likewise often underestimated. In the interconnected European power system, imports and exports influence CO₂ intensity. If a country exports a lot of renewable electricity in an hour, it can displace fossil generation in neighbouring countries. If it imports in scarcity hours, the effect depends on the power plants available there. National average values are well suited to political communication, but for operational decisions they are often too coarse.

Placing it in context: decarbonisation and system costs

CO₂ intensity is a useful indicator of decarbonisation, but not a complete yardstick for a functioning power system. It says little about grid congestion, firm capacity, storage needs, cost distribution or institutional responsibilities. Even a very low-emission electricity mix can place high demands on reserve capacities, grid expansion and flexibility. These requirements do not refute decarbonisation; they are part of its implementation.

For system costs, too, CO₂ intensity shows only a part. Low emissions per kilowatt hour can be associated with additional costs for grids, storage, redispatch or capacity mechanisms. Fossil generation can be flexible in the short term, but causes fuel costs, CO₂ costs and climate damage, which become visible to different degrees depending on the accounting framework. To understand an effect, one must look at the rule that produces it: market prices reflect short-term scarcities, grid charges finance infrastructure, emissions trading prices the direct emissions of the covered installations, and guarantees of origin allocate quantities of electricity in accounting terms.

CO₂ intensity describes the climate impact of electricity precisely, as long as the reference area, period, method, gas scope (CO₂ or CO₂e) and system boundary are stated. It is particularly informative together with load profiles, power plant dispatch, imports, flexibility and the fossil energy sources replaced. Without this context, a technically sensible indicator easily appears more precise than its accounting boundary allows.

Sources

Strom- und Wärmeversorgung in Zahlen — Umweltbundesamt; unbekannt. Abschnitt „Wie viel CO2 verursacht eine Kilowattstunde Strom im deutschen Strommix?“
Climate Change 16/2026: Entwicklung der spezifischen Treibhausgas-Emissionen des deutschen Strommix — Umweltbundesamt; 2026-03 (laut Dateipfad). Tabelle 1 sowie Abschnitte 1.3 und 2.1/2.2
CO₂-Emissionen pro Kilowattstunde Strom 2024 gesunken — Umweltbundesamt; unbekannt (Vorabschätzung zu 2024). Meldungstext
Richtlinie 2003/87/EG über ein System für den Handel mit Treibhausgasemissionszertifikaten (konsolidierte Fassung) — Europäische Union (EUR-Lex); 2024-03-01 (Konsolidierungsstand). Anhang I, Tätigkeit „Verbrennung von Brennstoffen“
IPCC AR5 WGIII, Chapter 7: Energy Systems — Intergovernmental Panel on Climate Change (IPCC); 2014. Abbildung 7.7 (direkte und Lebenszyklusemissionen in g CO₂e/kWh); Daten laut Kapitel in Anhang III, Abschnitt A.III.2

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Edited by eCMS on 11.11.2026 17:44