In May 2021, Britain's electricity averaged 178 gCO₂/kWh, in the moderate band (140–219 g). That is 11% cleaner than April 2021 (200 g), and 25% dirtier than May 2020 (142 g). Gas made 40.2% of the electricity generated, wind 18.8%, nuclear 15.3% and imports 10.9%; low-carbon sources together made 48.2%. The cleanest half hour was 58 g at 22:30 on Thursday 20 May, and the dirtiest 274 g at 21:00 on Saturday 1 May. Friday 21 May was both the cleanest day (90 g on average) and the windiest (51.4% wind).
Carbon intensity, day by day
Day by day, as a table
| Day | gCO₂/kWh | Low-carbon | Wind | Solar | Gas | Half hours |
|---|---|---|---|---|---|---|
| Sat 1 May | 237 | 33.4% | 3.7% | 6.0% | 56.0% | 48 |
| Sun 2 May | 228 | 35.6% | 4.5% | 6.6% | 53.8% | 48 |
| Mon 3 May | 141 | 59.3% | 33.1% | 2.8% | 31.9% | 48 |
| Tue 4 May | 97 | 72.1% | 47.2% | 5.0% | 21.8% | 48 |
| Wed 5 May | 171 | 53.0% | 24.8% | 5.8% | 38.5% | 48 |
| Thu 6 May | 214 | 42.6% | 14.8% | 5.7% | 48.8% | 48 |
| Fri 7 May | 223 | 39.7% | 10.3% | 6.5% | 47.3% | 48 |
| Sat 8 May | 126 | 60.1% | 36.6% | 1.6% | 26.5% | 48 |
| Sun 9 May | 111 | 65.8% | 38.9% | 5.5% | 22.3% | 48 |
| Mon 10 May | 116 | 64.0% | 36.1% | 5.9% | 25.3% | 48 |
| Tue 11 May | 174 | 49.3% | 19.6% | 6.0% | 38.6% | 48 |
| Wed 12 May | 212 | 40.2% | 11.9% | 5.6% | 46.2% | 48 |
| Thu 13 May | 234 | 34.9% | 8.0% | 3.3% | 50.7% | 48 |
| Fri 14 May | 244 | 32.6% | 6.0% | 3.6% | 52.3% | 48 |
| Sat 15 May | 228 | 33.9% | 9.4% | 4.2% | 51.2% | 48 |
| Sun 16 May | 210 | 36.9% | 8.3% | 5.6% | 48.6% | 48 |
| Mon 17 May | 231 | 35.8% | 8.2% | 5.1% | 50.7% | 48 |
| Tue 18 May | 228 | 37.2% | 7.2% | 5.9% | 50.4% | 48 |
| Wed 19 May | 222 | 37.1% | 6.2% | 6.7% | 51.8% | 48 |
| Thu 20 May | 142 | 57.2% | 32.0% | 2.4% | 32.8% | 48 |
| Fri 21 May | 90 | 72.6% | 51.4% | 2.5% | 21.0% | 48 |
| Sat 22 May | 143 | 56.9% | 27.3% | 5.2% | 33.0% | 48 |
| Sun 23 May | 114 | 63.1% | 31.6% | 4.3% | 25.0% | 48 |
| Mon 24 May | 145 | 55.5% | 24.6% | 5.3% | 33.1% | 48 |
| Tue 25 May | 147 | 55.0% | 24.6% | 4.1% | 33.8% | 48 |
| Wed 26 May | 183 | 45.6% | 15.2% | 5.1% | 42.5% | 48 |
| Thu 27 May | 199 | 41.1% | 6.6% | 7.6% | 46.4% | 48 |
| Fri 28 May | 220 | 37.0% | 3.7% | 5.0% | 50.7% | 48 |
| Sat 29 May | 193 | 41.1% | 4.7% | 7.6% | 46.2% | 48 |
| Sun 30 May | 160 | 50.3% | 8.7% | 10.3% | 37.3% | 48 |
| Mon 31 May | 141 | 56.2% | 17.4% | 10.1% | 32.5% | 48 |
What powered the grid
Each fuel's average share of the electricity generated in Great Britain in May 2021, with May 2020 beneath it for comparison. Imports count as one source, as NESO counts them.
Hours in each carbon band
- Very low (under 70 g): 16 hours, 2%
- Low (70–139 g): 188 hours, 25%
- Moderate (140–219 g): 337 hours, 45%
- High (220–299 g): 204 hours, 27%
Records in May 2021
- Cleanest half hour
- 58 g 22:30 on Thursday 20 May
- Dirtiest half hour
- 274 g 21:00 on Saturday 1 May
- Most wind
- 63.6% 02:00 on Tuesday 4 May
- Most solar
- 29.8% 13:00 on Monday 31 May
- Most low-carbon
- 85.9% 03:30 on Friday 21 May
- Least gas
- 12.5% 02:30 on Tuesday 4 May
- Cleanest day
- 90 g Friday 21 May
- Dirtiest day
- 244 g Friday 14 May
- Windiest day
- 51.4% wind Friday 21 May
- Sunniest day
- 10.3% solar Sunday 30 May
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.