Carbon intensity of electricity is the amount of carbon dioxide released to generate one kilowatt-hour of power. It is normally given in grams of CO2 per kilowatt-hour (gCO2/kWh): the lower the number, the cleaner the electricity on the grid at that moment.
How it is calculated
The National Energy System Operator (NESO) publishes a forecast and an estimate for Great Britain every half hour through its free Carbon Intensity API. It takes the output of each type of power station, multiplies it by an emission factor for that source and divides the total by all the electricity generated. Imports are counted using the generation mix of the country they come from.
| Source | Emission factor used by NESO (gCO2/kWh) |
|---|---|
| Coal | 937 |
| Gas, open cycle | 651 |
| Gas, combined cycle | 394 |
| Biomass | 120 |
| Imports from France | 53 |
| Wind, solar, nuclear, hydro | 0 |
These factors count only emissions at the point of generation, not from building the plant or mining its fuel.
Great Britain’s figures
By our own averaging of NESO’s half-hourly data (NESO does not publish an official annual figure), the grid ran at around 130 gCO2/kWh in both 2024 and 2025. The year-round average hides large swings. In 2025 the half-hourly figure ranged from about 30 gCO2/kWh on windy, sunny afternoons to almost 300 on still winter evenings, when gas plants fill the gap. Summer months are usually cleaner than winter ones: June 2025 averaged 97 gCO2/kWh, January 168.
With coal gone (2025 was the first calendar year in which no coal was burned for power, according to NESO), gas is now the main source of grid emissions.
Why it matters
Carbon intensity is the most direct measure of progress towards a clean power system. It also matters for households and businesses: running a dishwasher, charging an electric car or heating a hot-water tank when intensity is low cuts emissions without cutting use. Companies use grid intensity figures to report the emissions from the electricity they buy.
Read more: Live UK grid page.