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Emission factor

Emission Factor Carbon intensity electricity mix Carbon footprint

An emission factor indicates how many greenhouse gas emissions are attributed to a specific quantity of energy, a fuel input, a product or a service. In the electricity system, it is usually stated in grams per kilowatt-hour of electricity. If it refers only to carbon dioxide, the unit is g CO₂/kWh. If other greenhouse gases are also included, it is written g CO₂e/kWh. The kilowatt-hour is the quantity of energy; the emission factor is the climate impact attributed to it within a defined system boundary.

The designation CO₂e stands for CO₂ equivalents. Other greenhouse gases such as methane or nitrous oxide are converted, according to their climate impact, into an equivalent quantity of carbon dioxide. For electricity from coal, gas or oil, most of the emissions arise directly from combustion in the power plant. In addition there are emissions from upstream chains, for example from mining, gas extraction, methane leaks and transport. Wind, photovoltaics, hydropower and nuclear energy cause no or only very low direct CO₂ emissions during operation. For them, emissions arise mainly from manufacturing, construction, maintenance, fuel supply and decommissioning. For every emission factor it must therefore be clear whether it covers only direct emissions during operation or the entire life cycle.

Orders of magnitude: technologies and the German electricity mix

The differences between technologies are shown by the assessment in the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, IPCC (2014). Over the life cycle, it gives median values of about 820 g CO₂e/kWh for coal-fired power plants and about 490 g CO₂e/kWh for gas-fired power plants. For nuclear energy the figure is 12 g, for onshore wind 11 g and for offshore wind 12 g CO₂e/kWh. For utility-scale photovoltaics, about 48 g CO₂e/kWh is given. The direct emissions from combustion alone are about 760 g/kWh at the median for coal. For nuclear energy and wind they are zero. The ranges of the studies assessed are in some cases wide, for gas for example 410 to 650 g CO₂e/kWh and for nuclear energy 3.7 to 110 g. They depend, among other things, on location, capacity utilisation, lifetime, methane emissions in the gas upstream chain and the year of manufacture of the plants. More recent studies mostly arrive at lower values for modern photovoltaics.

For Germany, the Federal Environment Agency (UBA) calculates the emission factor of the electricity mix every year and publishes its development since 1990. According to the UBA's estimate, in 2025 an average of 344 grams of CO₂ were emitted in generation per kilowatt-hour of electricity consumed in Germany. In 2024 it was 353 g, in 2023 it was 379 g. In 1990 the value was still just over 760 g/kWh. Important for putting this in context: this well-known benchmark value counts only direct CO₂ emissions, that is, excluding methane, nitrous oxide and upstream chains. Values that include these components are reported separately by the UBA. They are higher. As reasons for the decline, the UBA cites the growing share of renewable energies, electricity consumption reduced as a result of economic stagnation, and net electricity imports. The agency therefore regards the decline from 2023 onwards only to a limited extent as a sign of lasting effective climate protection in the electricity sector.

Unit, system boundary and reference quantity

An emission factor is not a property of the electricity itself. Electricity from the socket cannot physically be sorted by its origin. The emission factor results from an allocation. For this it must be determined which generation plants are attributed to which consumption, which period applies, which gases and upstream chains count and which data source is used. An example: 1,000 kWh of household electricity correspond, using the UBA value for 2025, to roughly 344 kg of direct CO₂ emissions in calculation terms. With a life-cycle value in CO₂e or with an hourly value, a different figure results for the same quantity of electricity.

Several reference quantities are common for electricity. The average emission factor describes the emissions of total electricity generation in an area over a period, often a calendar year. It is suitable for many statistical climate balances because it assesses all consumption with a uniform factor. A distinction must be made as to whether it refers to domestic generation (production mix) or to consumption including electricity exchange with other countries. A time-variable emission factor, by contrast, describes how emission-intensive electricity is in a particular hour or quarter-hour. It becomes more important when flexible consumers such as heat pumps, electric cars, electrolysers or battery storage can shift their operation in time.

To be distinguished from this is the marginal emission factor. It asks which power plant or group of power plants changes its output when an additional kilowatt-hour is demanded or saved. For assessing an additional load, a load shift or an efficiency measure, this value can be more meaningful than an annual average. However, it is harder to determine. It depends on power plant dispatch, grid bottlenecks, fuel and CO₂ prices, weather, and imports and exports. In addition, a distinction is made between short-term marginal values in ongoing operation and long-term ones, which also take changes in the power plant fleet into account.

Distinction from CO₂ intensity, electricity mix and guarantees of origin

Emission factor and CO₂ intensity are often used almost synonymously. The UBA explicitly also refers to its indicator for direct CO₂ emissions per kilowatt-hour of electricity as an emission factor or specific emission. The difference lies more in usage than in physics. Emission factor sounds more like an accounting rule, CO₂ intensity more like a characteristic value of a generation mix or a technology. The method should therefore be stated for every numerical value.

The electricity mix describes from which energy sources or generation technologies electricity in an area is generated or consumed. The emission factor translates this mix into a climate impact per kilowatt-hour. A mix with a lot of lignite has a high emission factor, a mix with a lot of wind, photovoltaics, hydropower or nuclear energy a low one. Electricity imports, combined heat and power, storage losses and self-consumption can change the allocation. In combined heat and power, for example, it matters how the emissions of the fuel are divided between electricity and heat. It is also decisive whether a production mix, a consumption mix or a residual mix is considered.

Guarantees of origin are an accounting instrument. They certify that a specific quantity of electricity was generated from renewable sources. They do not change the physical flow of electricity in the grid, nor necessarily the power plant dispatch at a particular point in time. For companies' climate accounting, the Scope 2 Guidance of the GHG Protocol (2015) distinguishes two methods. The location-based method applies the average emission factor of the grid from which the electricity is drawn. The market-based method takes into account supply contracts, guarantees of origin, long-term power purchase agreements (PPAs) and suppliers' factors. For electricity without contractual evidence, it uses the residual mix. Companies operating in markets with such contractual instruments are to report both results separately under the Guidance. This is called dual reporting. The two figures answer different questions: one concerns the average supply at the location, the other contractual procurement.

Why the emission factor is relevant in the electricity system

The emission factor links technical electricity generation with climate accounting, investment decisions and policy steering instruments. Without it, one cannot calculate what emissions an electricity consumption causes, what an efficiency measure saves or what climate impact an electrification has. If an internal combustion engine is replaced by an electric car, a gas heating system by a heat pump or a fossil process by electric heat, the climate balance depends strongly on the electricity used. It also depends on how the electricity mix changes over the lifetime of the installation.

The emission factor is not a fixed natural value. It falls when renewable generation displaces fossil power plants, when low-emission firm capacity is available or when consumers run mainly at times of low CO₂ intensity. In the short term it can rise, for example when coal or gas power plants meet demand in hours with little wind and sun. The higher the share of renewable energies, the more the emission intensity fluctuates and the more important temporal allocation becomes.

For flexible consumers, therefore, annual consumption is not the only thing that counts. A heat pump that needs a lot of electricity mainly during cold periods of low wind and sun (Dunkelflauten) can have a different emission impact than a consumer that runs predominantly when there is a lot of wind or solar power. An electric car that usually charges at midday with high photovoltaic feed-in or at night with a lot of wind performs differently from one that regularly charges in the scarce evening hours. A flat annual factor reflects such differences only to a limited extent.

Typical misinterpretations

  • A single value is taken as the whole truth: An annual average is useful for many purposes. However, it does not explain which generation an additional load triggers. Anyone who assesses the effect of a load shift with the average value may underestimate or overestimate it.

  • CO₂ is confused with CO₂e: The well-known UBA value of 344 g/kWh for 2025 contains only direct CO₂ emissions. Values in CO₂e or including upstream chains are higher and not directly comparable.

  • A green electricity tariff is equated with emission-free consumption: A tariff with guarantees of origin allocates renewable electricity quantities in accounting terms. An additional climate impact arises only if the procurement incentivises new renewable generation, matches consumption in time or actually displaces fossil generation.

  • Electrification is automatically regarded as climate-neutral: Electric technologies often improve the balance because they are more efficient. A heat pump delivers several kilowatt-hours of heat from one kilowatt-hour of electricity, and an electric car uses the energy in the drive considerably better than a combustion-engine car. As a result, total final energy consumption can fall even if electricity consumption rises. How strongly emissions actually fall, however, depends on the emission factor of the electricity and on the fossil energy source replaced.

  • A high current value speaks against electrification: Electricity systems change, and an installation built today runs for many years. For its balance, the expected emission factors over the entire service life count, not just the value of a single year.

Technical and institutional contexts

The emission factor is closely linked to power plant dispatch. Power plants are dispatched not only according to their emissions but according to availability, marginal costs, grid restrictions, demand for balancing energy, fuel prices and market rules. The European emissions trading makes fossil generation more expensive in line with its CO₂ output and thus influences the order in which power plants are dispatched. However, grid bottlenecks can lead to low-emission generation being curtailed and fossil generation being ramped up elsewhere. The simply accounted electricity mix then deviates from the reality of grid operation.

Storage also changes the analysis. A battery storage system takes in electricity when charging and feeds it back in later. Because of conversion losses, it initially increases the quantity of electricity required. For climate protection it can nevertheless make sense if it shifts electricity from hours with a low emission factor into hours with a high one and thus reduces fossil peak load. A flat average factor is not sufficient for assessing it. What is decisive is the difference between the emission factors at the times of charging and discharging, with the losses having to be included in the calculation.

The same applies to flexibility. Flexible loads can reduce emissions if they shift their consumption to times of high renewable feed-in. However, they can also increase emissions if price signals, grid charges or technical control favour unfavourable times. The emission factor makes such effects visible. It does not replace an analysis of grid capacities, market rules and metering infrastructure.

For companies, municipalities and policy programmes, the choice of emission factor is a question of governance. Different standards and funding programmes permit different factors. This creates leeway but makes comparisons harder. At the GHG Protocol, a revision of the Scope 2 rules is currently under discussion which provides, among other things, for factors with finer spatial and temporal resolution. A climate balance appears precise if it reports figures with decimal places. It becomes reliable only when the system boundary, the greenhouse gases included, the period, the data source, the method and the purpose of the calculation are stated.

The emission factor is thus a measure of what climate impact is attributed to a kilowatt-hour in the electricity system. It does not describe where an individual electron comes from, and it does not replace consideration of capacity, grid operation or security of supply. It becomes meaningful only through the disclosed method: average or marginal, CO₂ only or CO₂e, direct or life-cycle, annual or temporally resolved, location-based or market-based. Without this information, the numerical value is easy to understand but technically incomplete.

Sources

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Edited by eCMS on 11.11.2026 19:45

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