In September 2021, Britain's electricity averaged 205 gCO₂/kWh, in the moderate band (140–219 g). That is 13% dirtier than August 2021 (182 g), and 5% dirtier than September 2020 (195 g). Gas made 41.8% of the electricity generated, wind 18.4%, nuclear 17.2% and imports 10.5%; low-carbon sources together made 46.2%. The cleanest half hour was 64 g at 04:00 on Thursday 23 September, and the dirtiest 302 g at 21:00 on Wednesday 15 September. The cleanest day was Monday 27 September (104 g on average); the windiest was Thursday 30 September (43.8% wind).
Carbon intensity, day by day
Day by day, as a table
| Day | gCO₂/kWh | Low-carbon | Wind | Solar | Gas | Half hours |
|---|---|---|---|---|---|---|
| Wed 1 Sep | 205 | 41.0% | 16.2% | 2.6% | 43.1% | 48 |
| Thu 2 Sep | 233 | 35.5% | 12.5% | 3.1% | 46.9% | 48 |
| Fri 3 Sep | 238 | 34.4% | 9.5% | 3.7% | 47.0% | 48 |
| Sat 4 Sep | 214 | 38.7% | 11.8% | 4.4% | 42.6% | 48 |
| Sun 5 Sep | 207 | 40.6% | 11.3% | 6.2% | 41.4% | 48 |
| Mon 6 Sep | 250 | 35.3% | 5.9% | 5.5% | 48.0% | 48 |
| Tue 7 Sep | 229 | 37.6% | 7.8% | 6.2% | 46.9% | 48 |
| Wed 8 Sep | 216 | 40.2% | 12.4% | 5.3% | 44.1% | 48 |
| Thu 9 Sep | 247 | 34.2% | 8.4% | 3.0% | 48.6% | 48 |
| Fri 10 Sep | 244 | 35.5% | 9.8% | 2.7% | 47.6% | 48 |
| Sat 11 Sep | 190 | 47.9% | 20.6% | 3.9% | 39.0% | 48 |
| Sun 12 Sep | 242 | 35.2% | 7.5% | 3.7% | 48.2% | 48 |
| Mon 13 Sep | 241 | 36.3% | 11.9% | 2.7% | 49.5% | 48 |
| Tue 14 Sep | 255 | 34.8% | 11.2% | 1.8% | 50.7% | 48 |
| Wed 15 Sep | 266 | 35.1% | 7.2% | 4.8% | 53.0% | 48 |
| Thu 16 Sep | 259 | 36.9% | 8.2% | 5.1% | 50.7% | 48 |
| Fri 17 Sep | 212 | 46.6% | 18.8% | 4.1% | 44.4% | 48 |
| Sat 18 Sep | 239 | 37.7% | 7.6% | 4.7% | 49.4% | 48 |
| Sun 19 Sep | 223 | 42.0% | 12.8% | 3.8% | 46.0% | 48 |
| Mon 20 Sep | 241 | 40.5% | 11.5% | 5.1% | 48.3% | 48 |
| Tue 21 Sep | 230 | 44.5% | 15.8% | 5.0% | 45.6% | 48 |
| Wed 22 Sep | 160 | 59.0% | 30.9% | 4.5% | 34.4% | 48 |
| Thu 23 Sep | 139 | 66.7% | 39.0% | 3.5% | 29.6% | 48 |
| Fri 24 Sep | 136 | 65.8% | 36.3% | 4.1% | 28.1% | 48 |
| Sat 25 Sep | 211 | 44.5% | 13.5% | 2.2% | 45.1% | 48 |
| Sun 26 Sep | 126 | 67.2% | 34.8% | 4.0% | 24.4% | 48 |
| Mon 27 Sep | 104 | 73.7% | 43.2% | 4.3% | 23.0% | 48 |
| Tue 28 Sep | 137 | 64.1% | 36.1% | 2.1% | 32.3% | 48 |
| Wed 29 Sep | 126 | 65.0% | 36.6% | 4.9% | 29.8% | 48 |
| Thu 30 Sep | 116 | 68.3% | 43.8% | 1.2% | 26.5% | 48 |
What powered the grid
Each fuel's average share of the electricity generated in Great Britain in September 2021, with September 2020 beneath it for comparison. Imports count as one source, as NESO counts them.
Hours in each carbon band
- Very low (under 70 g): 9 hours, 1%
- Low (70–139 g): 105 hours, 15%
- Moderate (140–219 g): 249 hours, 35%
- High (220–299 g): 358 hours, 50%
- Very high (300 g or more): 1 hours, 0%
Records in September 2021
- Cleanest half hour
- 64 g 04:00 on Thursday 23 September
- Dirtiest half hour
- 302 g 21:00 on Wednesday 15 September
- Most wind
- 58.5% 02:30 on Thursday 23 September
- Most solar
- 21.6% 13:30 on Sunday 5 September
- Most low-carbon
- 86.3% 03:00 on Tuesday 28 September
- Least gas
- 12.5% 23:30 on Monday 27 September
- Cleanest day
- 104 g Monday 27 September
- Dirtiest day
- 266 g Wednesday 15 September
- Windiest day
- 43.8% wind Thursday 30 September
- Sunniest day
- 6.2% solar Sunday 5 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 27 October 2026.