Low-emission generation · Glossary

Low-emission generation refers to electricity generation with low greenhouse gas emissions per unit of electricity generated. This is usually expressed in grams of CO₂ equivalent per kilowatt-hour (g CO₂e/kWh). This unit makes it possible to compare different greenhouse gases: methane, nitrous oxide and other climate-relevant gases are converted into the effect of carbon dioxide via their global warming potential. A time horizon of 100 years is customary. Which horizon and which conversion factors apply should be stated in every comparison.

The term does not stand for a particular technology but for a property of the emissions. Wind power, photovoltaics and, in most cases, hydropower and geothermal energy count as low-emission generation because they produce electricity without combustion. Their emissions arise mainly during manufacturing, construction and the use of materials. Not all of these plants are entirely free of operating emissions, however: depending on their location, reservoirs can release methane from decomposing organic material, and some geothermal reservoirs release dissolved CO₂. Nuclear energy is likewise classified as low-emission in life-cycle assessments, although it is not renewable. Fossil power plants with carbon capture and storage (CCS) can reduce their emissions considerably. How much depends on the capture rate, the fuel chain, methane losses and whether the CO₂ remains permanently stored.

Unit of measurement and system boundary

The central metric is emission intensity. It divides the greenhouse gas emissions caused by the electrical energy generated. A kilowatt-hour is a quantity of energy, not a power rating. A plant with low emission intensity can therefore still emit a lot in absolute terms if it generates a very large number of kilowatt-hours. Conversely, a plant with high intensity hardly matters in the annual balance if it runs only rarely. A comparison: a car's consumption in litres per 100 kilometres does not yet tell you how much fuel is used in a year. For that you also need the distance driven.

The system boundary is decisive for the assessment. Direct emissions arise at the site of electricity generation, for example when burning coal, gas or oil. Life-cycle emissions additionally include raw material extraction, manufacturing, construction, transport, fuel supply, maintenance, decommissioning and disposal. For wind and solar plants, the largest share arises in manufacturing, materials and installation. For gas-fired power plants, methane losses during extraction and transport are added to the CO₂ emissions at the stack. For nuclear energy, uranium mining, fuel fabrication, construction, decommissioning and waste treatment belong to the complete balance.

"Emission-free" is therefore usually imprecise. As a rule, what is meant is emission-free in operation, and that is a much narrower statement than emission-free over the entire life cycle. It is not sufficient for climate balances, support rules and investment decisions. Emissions in the supply chain and infrastructure do not disappear just because they do not arise at the power plant site.

Orders of magnitude over the life cycle

A life-cycle assessment by the United Nations Economic Commission for Europe (UNECE) from 2021 clearly shows the differences. It compares technologies in several world regions. The ranges depend, among other things, on capacity utilisation, methane leakage rates and the electricity mix used in manufacturing, and are not fixed values for individual plants:

Coal-fired power plants: about 751 to 1095 g CO₂e/kWh.
Natural gas combined-cycle power plants: about 403 to 513 g CO₂e/kWh, with CO₂ capture about 92 to 220 g CO₂e/kWh. Even CCS gas-fired plants are therefore not across the board as low-emission as wind or nuclear power in this study.
Wind energy: about 7.8 to 16 g CO₂e/kWh onshore and 12 to 23 g CO₂e/kWh offshore.
Photovoltaics: about 8 to 83 g CO₂e/kWh, depending on cell technology and location. Thin-film modules perform better than silicon-based modules.
Hydropower: strongly site-dependent, about 6 to 147 g CO₂e/kWh.
Nuclear energy: about 5.1 to 6.4 g CO₂e/kWh in the UNECE analysis.

The values show that low-emission technologies are typically one to two orders of magnitude below fossil generation without capture. However, they are not at zero. Other studies arrive at partly differing values depending on method, data basis and assumptions.

Distinction from renewable, low-carbon and climate-neutral

Low-emission generation is not the same as renewable energy. "Renewable" describes the origin of the energy: sun, wind, water, biomass and geothermal heat renew themselves on human time scales or are continuously available. "Low-emission" describes the climate impact per kilowatt-hour generated. A technology can thus be low-emission without being renewable; the most important example is nuclear energy. Conversely, renewable generation can also cause considerable emissions, for example when biomass comes from unsustainable land use, has long transport routes or displaces carbon sinks.

"Low-carbon" is narrower than "low in greenhouse gases" if only carbon dioxide is actually counted. For the climate, however, the entire greenhouse gas effect counts. For natural gas, the difference is considerable because methane has a particularly strong short-term effect. A pure CO₂ balance therefore makes gas-fired electricity look more favourable than a complete CO₂e balance including the methane losses of the upstream chain would justify.

Climate neutrality means something different: a net balance in which remaining emissions are offset by removals from the atmosphere. Whether and to what extent offsetting via certificates is recognised for this purpose depends on the respective set of rules and is disputed. A low-emission electricity source is therefore not automatically climate-neutral. Even in a power system with very low emissions, residual emissions remain important because they determine how much offsetting is needed elsewhere.

Significance in the power system

Low-emission generation is central to decarbonisation because electricity is meant to replace fossil energy sources in many areas. Heat pumps, electric vehicles, electrolysers, electric process heat and parts of industry shift energy consumption to electricity. This electrification reliably reduces greenhouse gas emissions only if the additional electricity comes predominantly from low-emission sources.

It is not only the annual quantity of electricity that counts. When electricity is generated also helps determine which power plants are displaced and which emissions are avoided. A kilowatt-hour of solar power at midday has a different effect than a kilowatt-hour on a dark, windless winter evening. For this reason, a distinction is made between the average emission intensity of the electricity mix and the marginal emission effect. The marginal view asks which power plant is ramped up or down in a particular hour by additional consumption or additional generation.

Low-emission technologies also contribute differently to security of supply. Wind and solar energy have very low emissions but generate depending on the weather. Hydropower with storage, biomass, geothermal energy, nuclear energy or fossil power plants with CO₂ capture can be scheduled more reliably, depending on their design. This matters for grid operation, residual load, storage needs and flexibility. A quantity of electricity with low emission intensity does not yet replace firm capacity if it is missing in the hours when the system needs power.

The same applies to economic assessment. Wind and solar plants have very low variable costs and thus change wholesale prices and the merit order of power plants. Costs for grid expansion, system services, storage, flexible demand or reserve capacity, by contrast, arise not at the level of the individual kilowatt-hour but in the overall system. A fair assessment therefore separates generation costs, integration costs and avoided emission costs.

Typical oversimplifications

"Low-emission" is often equated with "clean". "Clean", however, is not a technical term. It can refer to greenhouse gases, but also to air pollutants, land use, water demand, raw material chains, radioactive waste or interventions in ecosystems. A technology can perform very well on greenhouse gases and still have other environmental impacts. Conversely, a technology can emit few pollutants locally and still be inadequate in terms of climate policy.

A second error arises when accounting attribution and physical electricity flows are confused. Guarantees of origin document that a certain quantity of electricity was generated from a certain source. However, they do not change which power plants physically run in a particular hour. A company or a country can obtain green electricity on paper while a mixed European power plant fleet is operating at the same time. This matters for consumer products, corporate balances and political targets: it determines whether emissions actually fall or are merely booked differently.

Annual averages can also be deceptive. A power system can have a low average emission intensity over the year and still cause high emissions in individual hours when coal- or gas-fired power plants close the gap between demand and weather-dependent generation. The hourly view is therefore particularly important for flexible consumers, storage and electrolysers. Those who use electricity in low-emission hours can reduce the climate impact without more annual electricity having to be generated for that purpose.

For fossil generation with CO₂ capture, the actual residual emission is what matters. Capture does not always cover all CO₂ streams, requires additional energy and does not prevent methane emissions in the upstream chain. In addition, the climate impact depends on transport, storage, monitoring and liability rules. The UNECE values for gas-fired plants with CCS are clearly above those of wind or nuclear power. The technology can play a role in sectors that are hard to decarbonise. However, it does not automatically make fossil electricity generation low-emission in the same sense as a plant without combustion.

Institutional use: the example of the EU Taxonomy

In regulation, finance and industrial policy, the term is popular because it can describe climate impact across technology boundaries. Support programmes, taxonomies, tenders, electricity labelling, emissions trading and corporate reports need criteria to make technologies comparable. Where the thresholds lie helps determine which plants are considered future-proof, at what cost they obtain capital and which infrastructure receives political priority.

A concrete example is the EU Taxonomy for sustainable investments under Commission Delegated Regulation (EU) 2021/2139 in its consolidated version. For many electricity and heat generation activities, a technology-neutral threshold applies there: life-cycle emissions must be below 100 g CO₂e/kWh. For a substantial contribution to climate change mitigation, the regulation requires that these emissions be calculated and verified by independent third parties; depending on the section, this does not apply to all activities. With Delegated Regulation (EU) 2022/1214, nuclear energy and fossil gas were also added, subject to conditions, as transitional activities. For nuclear power plants, the 100 g life-cycle limit is likewise cited. For gas-fired power plants, in addition to this limit there is a time-limited alternative rule with its own requirements for direct emissions. The Taxonomy is a classification instrument for financial markets. It is neither an entitlement to support nor a permit requirement for power plants.

How contested this classification is shows in proceedings before the EU courts. On 10 September 2025, the General Court of the European Union dismissed Austria's action for annulment of Delegated Regulation (EU) 2022/1214 (Case T-625/22). In the Court's view, the Commission was entitled to assume that electricity generation from nuclear energy causes almost no greenhouse gases. According to its responsible ministry, Austria lodged an appeal with the Court of Justice in November 2025. The judgment is therefore not final. The outcome of the appeal should be checked separately before this statement is used.

This institutional role makes the term prone to conflict. If the limit is drawn too broadly, emission-intensive technologies receive a climate-friendly label. If it is drawn too narrowly, options drop out that contribute to emission reductions in certain system situations. How well the term works therefore depends on transparent system boundaries, measurable and verified emission values and a clear separation between climate impact, renewability, security of supply and other environmental consequences.

Conclusion

Low-emission generation is not a simple category for "good" power plants. The term makes the greenhouse gas impact of electricity production comparable. However, it requires operation, life cycle, temporal availability and integration into the system to be considered separately. Used precisely, it shows which electricity generation contributes to decarbonisation under which conditions. It also shows which questions emission intensity alone does not answer.

Sources

Life Cycle Assessment of Electricity Generation Options — United Nations Economic Commission for Europe (UNECE); 2021. Zusammenfassung der Ergebnisse (Key findings) und Kapitel zu fossilen, erneuerbaren und nuklearen Technologien
Delegierte Verordnung (EU) 2021/2139 (konsolidierte Fassung, Stand 08.01.2025) — Europäische Union, EUR-Lex; 2025-01-08. Anhang I, Abschnitt 4 (Energie), u. a. Tätigkeiten zu Kernenergie und fossilen gasförmigen Brennstoffen
Pressemitteilung Nr. 113/25: Urteil des Gerichts in der Rechtssache T-625/22, Österreich / Kommission — Gerichtshof der Europäischen Union, Referat Presse und Information; 2025-09-10. Pressemitteilung, Tenor und Begründungsübersicht
Österreich legt Rechtsmittel gegen Taxonomie-Urteil ein — Bundesministerium für Land- und Forstwirtschaft, Klima- und Umweltschutz, Regionen und Wasserwirtschaft (Österreich); 2025-11-20. Pressemitteilung

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Edited by eCMS on 11.11.2026 19:44