Understanding Carbon Intensity: Your Electricity's Carbon Footprint
Every kilowatt-hour of electricity you use carries an invisible carbon price. That price — measured in grams of CO2 equivalent per kilowatt-hour, or gCO2/kWh — is what we call carbon intensity. Understanding it is the first step to making smarter, greener energy decisions in Great Britain.
What Is Carbon Intensity?
Carbon intensity is a measure of how much greenhouse gas is emitted to generate a single unit of electricity. It combines two factors:
- The mix of power sources on the grid — gas, coal, nuclear, wind, solar, and hydro all carry different carbon costs
- The efficiency of those sources — how much CO2 is emitted per unit of useful energy produced
A grid powered entirely by nuclear and offshore wind might sit at 20–40 gCO2/kWh. A grid running heavily on gas and coal can exceed 400–500 gCO2/kWh. Great Britain today typically sits somewhere between 50 and 250 gCO2/kWh depending on the time of day and season.
Why Carbon Intensity Changes Throughout the Day
The GB grid is not static. Power demand rises and falls, and the grid operator — National Grid ESO — must balance supply and demand every 30 minutes. The carbon consequences of that balancing act are significant.
Low-carbon windows tend to occur:
- Midday on sunny days — solar generation peaks between 11am and 2pm BST, pushing carbon intensity down sharply
- Windy periods — offshore and onshore wind can supply over 50% of GB demand on gusty days, dramatically reducing reliance on gas
- Overnight in spring/autumn — wind generation often peaks at night, combined with low demand
High-carbon periods tend to occur:
- Winter weekday evenings (5–7pm) — demand peaks as people return home, solar output has already dropped, and gas peaking plants are called in
- Cold, still, overcast days — when wind and solar are simultaneously low and gas must cover the gap
Regional Carbon Intensity: Why Your Location Matters
One of the most overlooked aspects of the GB grid is that carbon intensity varies significantly by region. National Grid ESO and the University of Oxford publish regional estimates across 14 zones.
Scotland consistently has the lowest carbon intensity in Great Britain — often below 30 gCO2/kWh on windy days — thanks to its abundant onshore wind, offshore wind, and hydro resources. In the Scottish Highlands, carbon intensity can drop near zero.
Southern England tends to sit higher, relying more on interconnector imports from the Continent and gas generation. London and the South East have less local renewable generation relative to demand.
This geographic variation has real implications. Charging your EV in Edinburgh on a Tuesday afternoon produces a fraction of the emissions of charging it in London on a Monday evening.
How Is It Measured?
The Carbon Intensity API — jointly developed by National Grid ESO and the University of Oxford — uses a combination of live generation data from Elexon's BMRS and machine learning models to estimate both national and regional intensity in near real time.
The methodology considers:
- Direct emissions from fossil fuel generation (gas, coal, oil)
- Lifecycle emissions from low-carbon sources (a small amount for manufacturing solar panels, wind turbines, etc.)
- Interconnector imports — electricity imported from France (largely nuclear), Norway (largely hydro), and Belgium/Netherlands (a mix) carries a weighted carbon estimate
The result is updated every 30 minutes and includes a 48-hour forecast to help you plan ahead.
What Are "Green", "Yellow", and "Red" Bands?
The Carbon Intensity API classifies intensity into bands:
| Band | Range (gCO2/kWh) | Meaning |
|---|---|---|
| Very low | < 50 | Exceptional — wind or solar dominant |
| Low | 50–100 | Predominantly renewable |
| Moderate | 100–200 | Mixed; some gas in the mix |
| High | 200–300 | Significant gas generation |
| Very high | > 300 | Fossil fuels dominant |
On most days in 2025, GB hovers in the "low" to "moderate" range — a significant improvement from a decade ago, when coal often pushed it above 400 gCO2/kWh.
Practical Implications for Your Household
Carbon intensity isn't just an academic number. It has real, actionable consequences:
- Run your dishwasher or washing machine at midday on a sunny, windy day rather than at 6pm — you could cut the carbon cost of that cycle by 60% or more
- Charge your electric vehicle overnight when wind generation is high and demand is low, rather than after the evening commute
- Pre-heat or pre-cool your home during low-intensity windows if you have a heat pump or electric heating
- Avoid high-demand appliances during winter weekday evenings, when carbon intensity is typically at its highest
Small shifts in timing have a cumulative effect. The GB Power Insights Best Time to Use view provides a 48-hour forecast so you can plan ahead.
Great Britain's Carbon Intensity Trend
The GB grid's average carbon intensity has fallen dramatically since 2012, driven by:
- The closure of coal-fired power stations (the last UK coal plant closed in 2024)
- Massive deployment of offshore wind (now over 15 GW of installed capacity)
- Growth in rooftop solar (over 15 GW across Great Britain)
- Expanded interconnector capacity bringing in low-carbon nuclear power from France
In 2012, the average GB carbon intensity was around 500 gCO2/kWh. By 2023–24, it had fallen to around 200 gCO2/kWh on average — less than half. Progress continues.
Monitoring Carbon Intensity in Real Time
GB Power Insights pulls live regional carbon intensity data directly from the Carbon Intensity API, updated every 30 minutes. You can see:
- Live national intensity with a green/amber/red indicator
- Regional breakdown across all 14 GB zones — view the map
- 48-hour forecast to plan your energy use — see the forecast
Understanding carbon intensity transforms electricity from an invisible commodity into something you can actively manage. The data is free, public, and updated constantly — and GB Power Insights puts it at your fingertips.