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Glossary · Energy industry & energy transition

CO₂ equivalents

CO2 equivalents greenhouse gases Carbon footprint Emission Factor Climate neutrality life cycle analysis

CO₂ equivalents (CO₂e, also CO₂-eq.) are an accounting unit. They make it possible to express the climate impact of different greenhouse gases in a common unit. The figure answers the following question: How much carbon dioxide would have the same climate impact over a defined period as the considered quantity of another gas? With the usual global warming potential (GWP), what is compared is the radiative forcing summed up over the period, not the temperature change at a specific point in time. It is usually stated in kilograms or tonnes of CO₂e. The "e" stands for "equivalent" and not for a special form of carbon dioxide.

The concept is needed because greenhouse gases act very differently. A considerable share of emitted carbon dioxide remains in the atmosphere for centuries. Methane has a markedly stronger effect per kilogram but is broken down more quickly. The IPCC gives its atmospheric lifetime as around 12 years. Nitrous oxide (N₂O) has a high climate impact and remains in the atmosphere for a long time, about 109 years. Fluorinated gases, such as certain refrigerants or sulphur hexafluoride (SF₆) used in switchgear, have global warming potentials that in some cases reach four to five digits. If one stated only the mass of the gases in tonnes, these differences would remain hidden.

Conversion using global warming potentials

Conversion uses global warming potentials (GWP). The 100-year horizon is used most often, abbreviated GWP100. By definition, carbon dioxide has the value 1. For any other gas, the emitted quantity multiplied by the GWP gives CO₂e. According to the values of the IPCC Fifth Assessment Report (AR5), methane has a GWP100 of 28 and nitrous oxide of 265. One tonne of methane therefore corresponds to 28 tonnes of CO₂e. An illustrative example from building services: if 10 kilograms of the refrigerant HFC-134a, with a GWP100 of 1,300, escape, this corresponds to 13 tonnes of CO₂e.

The factors are reassessed with each IPCC Assessment Report because measurements and model assumptions continue to develop. The Sixth Assessment Report (AR6, 2021) distinguishes for the first time between fossil and non-fossil origin for methane. In the published main table, the 100-year horizon values are 29.8 for fossil and 27.0 for non-fossil methane. The difference arises because the oxidation of fossil methane releases additional CO₂ into the carbon cycle. For nitrous oxide, AR6 gives a value of 273. Anyone comparing CO₂e figures must therefore check whether AR4, AR5 or AR6 factors were used. Otherwise, deviations can arise solely from the factor basis.

Time horizon and legal basis

The time horizon chosen changes the result considerably. Short-lived gases with a high initial impact appear considerably more significant over a short horizon. This concerns methane above all: without chemical follow-up effects, AR6 gives methane a GWP of around 81 over 20 years, but around 28 over 100 years. For nitrous oxide, by contrast, both values are roughly the same. A CO₂e figure should therefore always state which time horizon and which factor basis underlie it. GWP100 is the widespread standard and creates comparability. However, this standard does not fully answer the question of how strongly short-lived emissions act over time. Alternative metrics exist for this, such as the global temperature change potential (GTP).

For official greenhouse gas reporting in the EU, the factor basis is laid down by law. Delegated Regulation (EU) 2020/1044 prescribes the AR5 GWP100 values for the Member States' national greenhouse gas inventories from reporting year 2021. It thereby follows the common metric agreed under the Paris Agreement. More recent EU legal acts also refer to this table for conversion into tonnes of CO₂-eq., for example Delegated Regulation (EU) 2026/285. In voluntary corporate reporting, by contrast, AR6 values are frequently used. Statutory inventories and corporate balances can therefore differ already in their factor basis. These statements apply only to the EU and only within the framework of the respective legal acts. Other regulatory frameworks may set their own factors.

Distinction from CO₂, carbon and climate neutrality

CO₂ equivalents describe neither a concentration in the atmosphere nor a quantity of energy. Nor are they the same as CO₂ emissions. CO₂ emissions refer to carbon dioxide actually emitted, for example from burning coal, oil or natural gas. CO₂e additionally covers all other greenhouse gases that are taken into account within the accounting boundary. An inventory can therefore show low direct CO₂ emissions and still show relevant CO₂e emissions. This is the case, for example, when methane escapes in the gas supply chain or when technical installations lose refrigerants.

Figures stated in "carbon" are also something different. Owing to the molar mass ratio of 44 to 12, one tonne of carbon corresponds to about 3.67 tonnes of CO₂. Anyone who mixes up the two reference quantities makes errors of almost a factor of four. In the energy sector, stating figures in tonnes of CO₂ or CO₂e is customary because it connects directly to emission factors and reporting obligations.

CO₂e differs fundamentally from climate neutrality. CO₂ equivalents measure emissions or emission reductions. Climate neutrality, by contrast, is an accounting claim: remaining emissions are to be offset through reductions, sinks or certificates. Without transparent information on CO₂e, accounting boundary, period and type of offsetting, such a claim can hardly be verified.

Relevance in the power system

In the power system, CO₂ equivalents make the climate impact of generation types, supply chains and consumption decisions comparable. At a coal-fired power plant, the relevant emissions arise predominantly directly at the stack. At a gas-fired power plant, upstream methane emissions from extraction, processing, transport and distribution are added to the direct CO₂ emissions. In gas engines, unburnt methane can also occur in the exhaust gas, the so-called methane slip. Wind energy, photovoltaics, hydropower and nuclear energy cause hardly any direct greenhouse gas emissions in operation. However, emissions arise during manufacturing, construction, maintenance, decommissioning and the provision of materials.

The accounting boundary is therefore decisive. An operational balance often covers only the direct emissions of a facility. A life cycle assessment additionally includes upstream and downstream processes. Both approaches answer different questions. For grid operation, what often matters is which emissions arise from current electricity generation. For investment decisions, it is additionally important what is incurred through construction, operation and decommissioning of a technology.

Companies include their electricity purchases in their climate balance via an emission factor in grams of CO₂e per kilowatt-hour. Within the framework of Scope 1, Scope 2 and Scope 3, purchased electricity falls under Scope 2. A national average, a supplier-specific value or a time-resolved value can be considered as the factor. This choice influences the result. An annual average conceals, for example, whether electricity was consumed in hours with a large share of renewable feed-in or with fossil marginal generation. With increasing electrification through heat pumps, electric vehicles, electrolysers and electric industrial processes, this distinction gains weight. CO₂e makes the effect visible in the accounts. However, the metric alone does not explain which power plants were displaced or additionally dispatched at a specific moment.

Typical misinterpretations

  • Reading CO₂e as an exact measurement: Most CO₂e values are calculated from activity data and factors, for example from fuel quantity, leakage rate, electricity consumption, material input or transport distance. They are only as reliable as the data, assumptions and accounting boundaries. Two differing figures for the same technology therefore need not be wrong.

  • Equating "low-emission in operation" with "emission-free": A photovoltaic system causes no combustion emissions during generation, but it does during manufacturing and transport. Conversely, a technology with low direct CO₂ emissions can perform considerably worse because of upstream greenhouse gases than an assessment at the site would suggest.

  • Assessing natural gas only via combustion CO₂: Natural gas then appears considerably more favourable than coal. If methane losses in the supply chain are included, the result shifts depending on origin, condition of the infrastructure, leakage rate, GWP factor and time horizon. Statements about climate benefits without information on methane and the accounting boundary remain incomplete.

  • Stating avoided emissions without a reference case: A "saving" is not a property of a facility but only arises from comparison with a reference. This reference can be an old coal boiler, an average electricity mix or a future, more renewable supply. It can be technically plausible, politically set or disputed.

  • Comparing values with different factor bases: A methane balance using AR5 factors (28) and one using AR6 values for fossil methane (29.8) are not directly comparable. If the factor basis changes, time series must also be recalculated.

Institutional and economic significance

CO₂ equivalents form a basis for emission inventories, climate reporting, funding programmes, product declarations and procurement rules. In the European emissions trading system, CO₂ from combustion dominates at most energy installations. For other covered gases such as nitrous oxide or perfluorocarbons in certain industrial processes, conversion into CO₂e takes place. Companies report direct emissions, emissions from purchased energy and further emissions in the value chain. For this they use standardised categories. In this way, climate impact becomes a verifiable quantity in regulation, financing and supply chain management.

The economic effect depends on which rule draws on the unit. A CO₂ price changes fuel costs and the merit order of power plants. A limit value for emissions per kilowatt-hour can steer investments. Proof for green hydrogen presupposes that it is specified which quantities of electricity were used and which emissions are attributed to them. As soon as CO₂e decides on costs, approval or crediting, it is no longer merely a scientific calculation quantity but also an administrative one. Clean accounting rules are therefore important. With guarantees of origin, the contractual allocation can deviate from the physical delivery. Flexible consumers need data of finer temporal resolution than annual averages. For storage, it must be regulated which emissions are attributed to the electricity charged and discharged.

CO₂ equivalents make greenhouse gases, technologies and supply chains comparable. However, they do not replace the examination of time horizon, factor basis, accounting boundary, data quality and reference case. Used precisely, CO₂e is not a label for climate-friendliness. It is a calculation quantity that makes climate impact comparable and at the same time discloses the assumptions on which this comparison rests.

Sources

Related glossary entries

Edited by eCMS on 11.11.2026 17:37

© 2026 Erik Schmidt. All rights reserved. Quotations with a link to the source are welcome.