In September 2025, Britain's electricity averaged 112 gCO₂/kWh, in the low band (70–139 g). That is 10% cleaner than August 2025 (124 g), and 8% cleaner than September 2024 (122 g). Wind made 38.6% of the electricity generated, gas 23.9%, imports 11.4% and nuclear 10.8%; low-carbon sources together made 64.6%. The cleanest half hour was 34 g at 13:00 on Friday 12 September, and the dirtiest 260 g at 20:30 on Sunday 28 September. Monday 15 September was both the cleanest day (54 g on average) and the windiest (64.5% wind).
Carbon intensity, day by day
Day by day, as a table
| Day | gCO₂/kWh | Low-carbon | Wind | Solar | Gas | Half hours |
|---|---|---|---|---|---|---|
| Mon 1 Sep | 67 | 78.4% | 55.1% | 7.5% | 14.2% | 48 |
| Tue 2 Sep | 138 | 57.9% | 29.3% | 6.5% | 29.9% | 48 |
| Wed 3 Sep | 107 | 69.8% | 38.5% | 5.6% | 18.5% | 48 |
| Thu 4 Sep | 125 | 62.9% | 31.9% | 7.6% | 25.7% | 48 |
| Fri 5 Sep | 141 | 55.0% | 20.9% | 9.8% | 29.7% | 48 |
| Sat 6 Sep | 79 | 70.1% | 40.2% | 9.8% | 13.9% | 48 |
| Sun 7 Sep | 66 | 78.9% | 57.5% | 5.2% | 12.2% | 48 |
| Mon 8 Sep | 127 | 60.0% | 27.0% | 10.8% | 28.2% | 48 |
| Tue 9 Sep | 143 | 56.6% | 28.0% | 8.3% | 33.7% | 48 |
| Wed 10 Sep | 63 | 80.1% | 56.5% | 4.8% | 12.1% | 48 |
| Thu 11 Sep | 56 | 80.6% | 59.0% | 7.6% | 11.2% | 48 |
| Fri 12 Sep | 63 | 80.2% | 57.0% | 8.4% | 13.3% | 48 |
| Sat 13 Sep | 76 | 72.9% | 46.3% | 9.2% | 14.8% | 48 |
| Sun 14 Sep | 93 | 66.6% | 42.1% | 3.7% | 17.9% | 48 |
| Mon 15 Sep | 54 | 83.2% | 64.5% | 6.4% | 10.8% | 48 |
| Tue 16 Sep | 73 | 76.7% | 56.2% | 7.4% | 15.6% | 48 |
| Wed 17 Sep | 74 | 75.4% | 54.3% | 3.1% | 14.0% | 48 |
| Thu 18 Sep | 83 | 75.9% | 57.3% | 4.4% | 18.5% | 48 |
| Fri 19 Sep | 114 | 63.3% | 38.6% | 7.0% | 25.0% | 48 |
| Sat 20 Sep | 120 | 60.2% | 36.1% | 3.0% | 25.7% | 48 |
| Sun 21 Sep | 76 | 74.4% | 49.3% | 9.7% | 15.5% | 48 |
| Mon 22 Sep | 122 | 60.4% | 31.6% | 10.0% | 25.5% | 48 |
| Tue 23 Sep | 181 | 45.6% | 16.7% | 8.3% | 41.3% | 48 |
| Wed 24 Sep | 161 | 51.5% | 23.6% | 7.3% | 34.0% | 48 |
| Thu 25 Sep | 122 | 61.4% | 31.0% | 9.8% | 24.2% | 48 |
| Fri 26 Sep | 139 | 57.3% | 31.5% | 4.7% | 30.4% | 48 |
| Sat 27 Sep | 122 | 60.6% | 36.9% | 4.5% | 26.5% | 48 |
| Sun 28 Sep | 190 | 41.0% | 11.6% | 7.8% | 41.1% | 48 |
| Mon 29 Sep | 215 | 36.0% | 9.0% | 7.0% | 51.5% | 48 |
| Tue 30 Sep | 182 | 46.1% | 19.8% | 7.2% | 42.8% | 48 |
What powered the grid
Each fuel's average share of the electricity generated in Great Britain in September 2025, with September 2024 beneath it for comparison. Imports count as one source, as NESO counts them.
Hours in each carbon band
- Very low (under 70 g): 185 hours, 26%
- Low (70–139 g): 317 hours, 44%
- Moderate (140–219 g): 193 hours, 27%
- High (220–299 g): 25 hours, 3%
Records in September 2025
- Cleanest half hour
- 34 g 13:00 on Friday 12 September
- Dirtiest half hour
- 260 g 20:30 on Sunday 28 September
- Most wind
- 73.3% 00:30 on Tuesday 16 September
- Most solar
- 33.8% 13:30 on Monday 8 September
- Most low-carbon
- 89.2% 13:30 on Monday 15 September
- Least gas
- 6.2% 12:00 on Friday 12 September
- Cleanest day
- 54 g Monday 15 September
- Dirtiest day
- 215 g Monday 29 September
- Windiest day
- 64.5% wind Monday 15 September
- Sunniest day
- 10.8% solar Monday 8 September
About these figures
GridMonkey records NESO's carbon intensity and generation mix for Great Britain every half hour: the actual figure where NESO has published one, otherwise its forecast. They are added up by UK calendar day and month. Percentages are shares of the electricity generated, imports included, so the fuels add up to 100%. Low-carbon means wind, solar, nuclear, biomass and hydro. The carbon bands are GridMonkey's: very low under 70 g, low 70–139 g, moderate 140–219 g, high 220–299 g and very high 300 g or more, per kWh. Northern Ireland has its own grid and isn't included.
To see the grid now, open GridMonkey. For the wind farms that ran hardest this week, see the wind farm league table.
Data: NESO Carbon Intensity API (carbonintensity.org.uk), licensed under CC BY 4.0, recorded and summarised by GridMonkey. Figures as of 19 October 2026.