Britain’s Grid Was Nearly Twice As Clean At Lunchtime As At 4am, New Figures Show
At half past eleven in the morning on 19 August 2026, every kilowatt hour of electricity used in Great Britain carried just 80 grams of carbon dioxide with it. Sixteen and a half hours later, at 4am, that same kilowatt hour cost the atmosphere 189 grams. That is more than double, inside a single day, without anyone building a new power station or turning off a factory. According to National Grid ESO’s Carbon Intensity API, the average across the whole day landed at about 148 grams of CO2 per kWh, a figure that hides just how much the cleanliness of Britain’s power supply moves depending on the time you switch the kettle on.
Why the clock matters as much as the weather
The gap between 80 grams and 189 grams is not a rounding error, it is the difference between a grid running mostly on sunshine and one leaning on whatever is available overnight. Carbon intensity measures how much CO2 is released for every kilowatt hour of electricity generated, and it moves in step with the mix of power sources feeding the grid at any given moment. When the sun is high and solar panels are working hard, intensity falls. When the sun has gone down and demand for lights and heating creeps up, the grid has to lean on sources that produce more carbon per unit, and the number climbs.
This matters for anyone trying to be sensible about when they use power, not just for the grid operator. A washing machine cycle run at 11:30 draws on a supply that, according to National Grid ESO’s Carbon Intensity API, was running at 80 grams of CO2 per kWh that day. The same cycle run at 4am draws on a supply running at 189 grams. The appliance uses the same electricity either way, but the carbon cost attached to it is very different depending on the hour.
None of this is about people being wasteful or careless. Most households simply use power when they need it, and overnight demand exists for good reasons, from fridges to hospitals to factories running shifts. But the scale of the swing shown in the data is a reminder that Britain’s electricity supply is not a single, constant thing. It changes hour by hour, and increasingly the cleanest hours are the ones when the sun is out.
What was actually powering the grid
The National Grid ESO Carbon Intensity API also breaks down what was generating the electricity, and the latest reading tells a clear story about where Britain’s power came from. Solar led the mix at 23.3%, the single largest contributor. Imports, electricity brought in from interconnectors to other countries, came next at 22.1%. Wind supplied 21.4%, and nuclear contributed 14.1%.
Together, solar, wind, nuclear and the other low-carbon sources tracked by National Grid ESO, which include hydro and biomass, made up about 65% of the latest generation mix. That means roughly two thirds of the electricity flowing through the grid at that moment came from sources that produce little or no carbon dioxide during generation. The remaining share came from imports and other sources not counted in that low-carbon total.
The prominence of imports in the mix, at 22.1%, is worth pausing on. It shows that a meaningful slice of Britain’s electricity in that snapshot did not come from a power station on British soil at all, but arrived via undersea or cross-border cables from elsewhere. This is a normal and long-standing part of how the grid works, but it means the carbon intensity of Britain’s electricity is not entirely in Britain’s hands. It also depends on what is happening, and what is generating power, in the countries on the other end of those interconnectors.
Reading the daily swing correctly
It would be easy to look at the 80 gram figure at 11:30 and assume the grid is now reliably clean, or to look at the 189 gram figure at 4am and assume it has gone backwards. Neither reading is right. The daily average of about 148 grams of CO2 per kWh, as recorded by National Grid ESO’s Carbon Intensity API, is the more honest summary of the day as a whole. It sits between the two extremes, reflecting the hours when solar and wind were plentiful and the hours when they were not.
What the swing does show is that solar generation in particular has a very direct, very visible effect on how clean the grid is at any given hour. The cleanest point of the day, 11:30, falls squarely in the window when solar panels are producing at or near their peak. The dirtiest point, 4am, falls in the window when solar contributes nothing at all and the grid has to rely more heavily on other sources to meet overnight demand.
This pattern is not a one-off curiosity for a single day in August. It reflects a structural feature of a grid that now draws heavily on weather-dependent generation. Solar and wind can supply a large share of the day’s electricity when conditions are favourable, as they clearly were in this snapshot, but that share is not constant across 24 hours in the way that output from a nuclear plant or a gas-fired station tends to be.
What this means for readers
The practical takeaway from these figures is straightforward. If cutting the carbon footprint of your electricity use matters to you, timing has a real, measurable effect, at least on the day the figures were recorded. Running power-hungry appliances closer to the middle of the day, when the National Grid ESO Carbon Intensity API recorded the grid at its cleanest, has a genuinely different impact than running them in the early hours, when the same data showed it at its most carbon-intensive.
More broadly, the figures are a useful corrective to the idea that Britain’s electricity supply has a single, fixed level of cleanliness. It does not. It moves with the sun, the wind, and the flows through the interconnectors, hour by hour. Solar’s 23.3% share and wind’s 21.4% share in the latest mix show how central weather-dependent generation has become to keeping the lights on, while the 22.1% drawn from imports and 14.1% from nuclear show how much of the rest still depends on sources outside that weather system, or outside the country altogether.
As more of Britain’s electricity comes from solar and wind, expect the gap between the cleanest and dirtiest hours of the day to remain a feature of the grid rather than a temporary quirk. Whether that gap narrows or widens will depend on how much low-carbon capacity is added, and how much the grid continues to lean on imports and other sources when the sun goes down and the wind drops. For now, the data from National Grid ESO’s Carbon Intensity API tells a simple story, that when Britain uses its electricity matters almost as much as how much it uses.
The data
| Fuel | Share of generation |
|---|---|
| Solar | 23.3% |
| Imports | 22.1% |
| Wind | 21.4% |
| Nuclear | 14.1% |
| Gas | 11.1% |
| Biomass | 5.9% |
| Other | 1.6% |
| Hydro | 0.6% |
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Data: National Grid ESO Carbon Intensity API. View the dataset.
Sam Allcock is the founder and editor of Morrow Tech. He has spent over a decade in digital publishing and writes about the technology worth your attention, from AI to everyday gadgets.