Explainer
Carbon intensity, explained
What grid carbon intensity actually measures, why GB's number swings from under 50 to over 300 gCO2/kWh in a single day, and how to use it to cut your real emissions.
Right now on the GB grid
Generating 24.2 GW, 47.0% of it wind, at 76 gCO₂/kWh (low), importing 3.0 GW over the interconnectors.
As of 2026-08-25 00:43:40 UTC · watch it live
Carbon intensity is the emissions price of a unit of electricity: how many grams of CO₂ were released to make each kilowatt-hour. Great Britain’s number is not a constant - it is a live signal that can swing from under 50 gCO₂/kWh on a windy spring afternoon to over 300 on a still winter evening. Right now it is here.
Why it moves
The grid meets demand with whatever mix is available, and each fuel carries a different footprint. Wind, solar, hydro and nuclear are effectively zero-carbon at the point of generation. Biomass is counted as low-carbon in official figures. Gas sits around 350-400 gCO₂/kWh - and since coal ended in 2024, gas is the number’s main driver: when the wind drops, gas fills the gap and the intensity climbs.
That is the whole story of the daily swing. The yearly story is bigger: the grid averaged around 500 gCO₂/kWh in 2012 and closer to 125 in 2025 - one of the fastest decarbonisations of any grid on Earth. The extremes live on the records board.
Where the number comes from
The official figure is published by the Carbon Intensity API - a NESO and University of Oxford collaboration - every half hour, with a 48-hour forecast. It is embedded-inclusive: rooftop solar and small wind that never touch the transmission grid are estimated in, which matters because about 29% of GB capacity is embedded. Regional figures exist too, but are a model, not a measurement - our region pages label them that way.
Using it
Because the forecast is public, you can shift flexible electricity use - EV charging, laundry, a hot-water tank - into the cleanest window of the day. The greenest upcoming slot is computed here, straight from the official 48-hour forecast. At the margin, that shift means real gas not burned.
One honest caveat: carbon intensity measures generation, not your supplier’s tariff. A “100% renewable” tariff doesn’t change what the grid is physically doing when you flip a switch - the live number is the same for everyone plugged into it.
Common questions
- What is a good carbon intensity number?
- Under 100 gCO2/kWh is genuinely clean for the GB grid (the official index calls under ~50 'very low'). Above 250 means gas is dominating the mix. The all-time greenest half-hour is on the records board.
- Does using electricity at low-carbon times really cut emissions?
- Yes, at the margin. Shifting flexible use into windy or sunny hours means less gas burned to serve you. It is one of the few climate actions where timing alone, with no sacrifice, has a real effect.
More explainers
- Embedded generation: the third of the grid you can't see
- How batteries balance the grid
- How interconnectors work
- How the UK electricity grid works
- Is the UK still burning coal for electricity?
- The UK coal phase-out: a timeline
- What is curtailment - and why do wind farms get paid to stop?
- Why does the grid run at 50 Hz?
- Why electricity prices go negative