This is part 1 of a series. Next, part 2: The Heat Grid Is the Big One
Wind and Solar Power
The biggest growing source of power over the last 15 years, wind and solar power are both totally dependent on short-term weather. They are not firm in a traditional sense. You can't bank on them being able to generate beyond the forecastable horizon of the weather.
With the UK's geography and planning system, wind also tends to be built further north, creating an additional dependency on the successful expansion of grid transmission capacity. If it's "too" windy in Scotland relative to the rest of Britain and demand, some of the wind power generated will have to be curtailed and substituted for elsewhere.
Wind and solar do have some positive attributes from a reliability perspective. They are inherently modular, comprising individual turbines (up to 10MW) and arrays of panels. Compared to a typical thermal power plant unit of 500MW, they can often operate without onsite staff, and individual turbines/arrays can fail without causing a wider outage.
While bad weather can severely limit wind and solar generation, at certain times it is possible to be much more confident of a minimal generation output. For example, midday solar generation even on an overcast or rainy day will hit a certain minimal level, even if this is just a fraction of the potential. The diversity of wind farm locations similarly means that there is always some wind generation somewhere. And if you consider wind and solar as part of a future energy system with more storage and flexibility, what becomes more relevant for firmness is the rolling average of the combined national wind/solar portfolio over time.
Figure 1 — firmness is a function of the averaging window
Over three complete years the combined GB wind and solar fleet averaged 11.2 GW. Its worst half hour was 0.2 GW — 2% of that. Its worst seven days was 3.3 GW, 29%. Its worst thirty days was 6.7 GW, 60%. Nothing about the fleet changed between those numbers; only how long you are allowed to wait.
The right-hand panel asks the same question one season at a time, each normalised to its own mean, and the shapes differ in a way that names which technology is doing the work:
30 minutes — winter: 2% · spring: 5% · summer: 4% · autumn: 4%
1 day — winter: 10% · spring: 28% · summer: 33% · autumn: 15%
7 days — winter: 44% · spring: 51% · summer: 57% · autumn: 29%
30 days — winter: 76% · spring: 72% · summer: 77% · autumn: 68%
season mean — winter: 13.0 GW · spring: 10.5 GW · summer: 10.0 GW · autumn: 11.3 GW
Solar buys firmness by the day; wind buys it by the month. Summer has the lowest mean output of the four seasons and the highest one-day floor — a third of its average, because the sun turns up every single day — but that daily floor buys nothing extra at a week or a month, so the curve flattens. Winter is the mirror image: nothing is guaranteed daily, so its one-day floor is 10% — summer's daily floor is three and a half times winter's — but no winter month in three years has been becalmed and its thirty-day floor is the best of the four at 76%. Autumn is the worst place to be at a week, 29%, which is the October 2025 wind drought that figure 7 also picks out. By thirty days the seasons have converged: all four sit inside 68–77%.
Figure 2 — solar fails on a timetable, wind fails on the weather
That difference in duration has a mechanism, and it is not that winter is windier. It is that the two technologies fail on different clocks.
Solar has a guaranteed daily zero. Every summer night output falls back to the wind alone, and it does so on schedule — 6.7 GW at 04:00 against 15.3 GW at half past twelve. Wind has no daily anything. Average winter output at 04:30 and at noon differ by 6%: the clock tells you almost nothing about how windy it is going to be.
Put as a variance decomposition — how much of each season's spread is explained by its own average daily profile:
winter — the clock explains: 4% · wind swings across the day: 6% · solar above 1 GW: 6 h a day
summer — the clock explains: 36% · wind swings across the day: 20% · solar above 1 GW: 12.5 h a day
Which is why the shortfalls have different shapes. Summer's are frequent, regular and short — nine in ten are over inside 21 hours, and the worst in three years ran 3.9 days. Winter's are rarer but open-ended: nine in ten inside 2.4 days, worst 6.6. Autumn is worse than either at 10.4 days — the October 2025 drought again.
Storage that empties and refills every night is a different asset from storage that has to ride out a week, and the same fleet needs both, in different seasons.
One check worth stating, because these are metered series and metering is net of curtailment — which is itself diurnal. Against the transmission fleet's own 2025 availability:
winter — delivered swing: 7.8% · available swing: 3.5% · curtailment peaks: 06:00, 1.5 GW
summer — delivered swing: 17.0% · available swing: 17.7% · curtailment peaks: 19:00, 1.3 GW
Winter's small daily shape is mostly an artefact — the network bidding wind off overnight when demand is lowest. The resource underneath is flatter still. Summer's is weather: delivered and available swing by the same amount and peak in the same late afternoon, which is the ordinary onshore diurnal cycle as daytime heating mixes momentum down. So the 4% figure overstates how much the winter clock tells you, and the 36% is real.
Figure 3 — solar's floor is real, and it is a summer floor
Across three Junes, the worst midday half hour GB solar ever produced was 3.6 GW — 41% of the median June midday. Whatever the weather, the middle of a June day is worth something. December's floor is 0.3 GW, 14% of an already small median.
Figure 4 — portfolio diversification buys a great deal, but not a floor
Portfolio diversification cannot add energy, so it can only move it — and the left panel is that trade. The pooled curve runs below the median farm for the first 28% of the year and above it for the other 72%. The fleet is never all-out at once, which costs it the top: its best half hour is 90% of its parts' summed peak, while every individual farm reaches 100% of its own. It is almost never all-off at once, which buys it the bottom: at the 90th percentile of the year the median farm is producing nothing and the fleet still has 10% of its peak.
The right panel counts the same fact differently: the median single farm sits below 5% of its own peak for 24% of the year, the pooled fleet for 3%. But at 09:00 on 22 January 2025 the whole fleet fell to 0.1% of its peak, out of 23.6 GW. Britain is not wide enough to be its own portfolio.
Output here is availability, not metered — a farm that was paid to stop is not a farm with no wind. On metered output the median farm reads 30% rather than 24%, and Moray East, half of whose year was bid off, looks becalmed 39% of the time against a true 23%. Curtailment is a separate argument and gets the next figure.
Figure 5 — in a national calm, what matters is the distance, not the site
If diversification is what buys the floor, which parts of the portfolio are actually doing the diversifying? Take the 10% of 2025 when the pooled fleet was at its lowest — the fleet averaged 7.0% of peak against 40.4% over the year — and ask what each unit was doing.
Start with the thing that is true of all of them. In a national calm the fleet keeps 17% of its own normal output, and there is no farm in Britain that keeps more than 36%. So nothing in the table below holds up in any absolute sense: the entire spread runs from losing two thirds of your normal output to losing nine tenths of it. The question is only which farms are still contributing something when the rest have effectively stopped.
On that, it is not simply the windy sites. Annual load factor explains only a third of how a farm performs in a calm. The rest is geography, and the pattern is the extremities of the map against the middle:
Viking, Shetland — annual: 44.2% · in a calm: 15.8% · keeps: 36% · · worst in a calm: Harestanes, Dumfriesshire · annual: 22.6% · in a calm: 2.1% · keeps: 9%
Rampion, off Sussex — annual: 41.9% · in a calm: 14.2% · keeps: 34% · · worst in a calm: Griffin, Perthshire · annual: 21.3% · in a calm: 2.5% · keeps: 12%
Neart na Gaoithe, off Fife — annual: 41.3% · in a calm: 11.7% · keeps: 28% · · worst in a calm: Whitelee, south of Glasgow · annual: 23.7% · in a calm: 2.9% · keeps: 12%
Galloper, off Suffolk — annual: 48.6% · in a calm: 11.4% · keeps: 23% · · worst in a calm: Black Law, Lanarkshire · annual: 21.3% · in a calm: 3.1% · keeps: 15%
East Anglia One, off Suffolk — annual: 48.8% · in a calm: 9.7% · keeps: 20% · · worst in a calm: Crystal Rig II, Lammermuirs · annual: 26.6% · in a calm: 3.4% · keeps: 13%
Read the right-hand block as a map and it is one place: the Southern Uplands and the central belt, every one of them within about 80 miles of Glasgow. The left-hand block is Shetland, Sussex, Fife and Suffolk — the corners.
And 15.8% is not a good outcome, which is the other half of reading that table honestly. Viking is the best-sited wind farm in Britain for exactly this problem and in a calm it runs at about a seventh of its nameplate. Take the counterfactual seriously — rebuild every farm in the country to Viking's standard, ignore that its site is unrepeatable — and the units this figure measures — those above 100 MW, 17.2 GW of the fleet's 23.6 — go from 1.3 GW in the median calm half hour to 2.7 GW, against a winter peak near 45 GW. The prize for getting the geography perfectly right is worth about one and a half gigawatts. It is not nothing, and it is not a floor.
And the corners are the expensive end of the fleet: four of the best five are offshore and all five of the worst are onshore Scotland, with AR7 clearing offshore wind at £91.20/MWh against onshore at £72.24, before transmission charges that run the same way. Whether a premium like that is repaid by about 1.5 GW of avoided firm capacity is not obvious — the less so because, as a later part will show, you need very nearly as much gas standing behind either fleet.
Moray East is the case that makes the point, because it is the one that cannot be explained by its site being poor. It has one of the best annual load factors in the country at 44.0%, and in a national calm it delivers 6.9% — it keeps 16% of its normal output where Viking, on a near-identical annual figure of 44.2%, keeps 36%. Beatrice, its neighbour in the Moray Firth, does the same: 41.3% across the year, 6.7% in a calm.
What separates them from Viking is not the turbine, and it is not how much wind the site gets — the annual means are within two tenths of a point of each other. It is how that wind is distributed. Shetland sits 250 km further north, which is far enough to fall outside the blocking high that becalms the mainland: same average, different tail. And the tail is what a firm system is sized against.
And the worst retention in the country belongs to good sites, not bad ones. Race Bank off Norfolk averages 42.6% across the year and keeps 12% of it in a calm; West of Duddon Sands 44.1% and 13%; Triton Knoll 39.0% and 12%; Ormonde 34.8% and 11%. A farm can be among the windiest in Britain on an average day and among the least useful on the day it is needed.
So there is a real effect here, and it is worth being precise about what it is — because the obvious reading, that Britain should buy the corners and stop building in Lanarkshire, is only about half right.
The corners do decorrelate. Across 2025 the average pair of British wind farms moved together at +0.48. In the calm decile that falls to +0.09, and Viking and Rampion — Shetland and Sussex, just over 1,000 km apart, about as far as two British wind farms can get from each other — go from +0.07 across the year to −0.14 in a calm. Textbook diversification: when the fleet is struggling, the units stop moving as one.
And it buys almost nothing. The fleet still fell to 0.11% of peak at 09:00 on 22 January. In the median calm half hour 78% of the fleet is below a tenth of its peak at the same moment, and in 79% of them there is not a single farm anywhere in the country above half of its. In the worst half hour of the year, the best-performing wind farm in Britain — out of forty-five, spread over a thousand kilometres — was managing 4.2%.
Which is the resolution of the paradox, and the thing the geography is really telling us. The farms genuinely do become independent of each other in a calm. But independence at 7% of peak is independence between small numbers, and small numbers that are uncorrelated still sum to a small number. What a firm system needs is not for the farms to stop moving together; it is for somebody to be windy — and in a British calm nobody is, because the high-pressure system that causes it is bigger than the country. Britain is roughly a thousand kilometres from end to end, and that is not far enough to hold two weathers.
So the marginal value of a wind farm to a firm system is indeed how far it sits from everything already built, and Britain has been buying the wrong thing on that measure — the cheapest sites are in the middle of the existing cluster. But the ceiling on what the right answer buys you is low, and it is set by the size of the island. Which is why the search for a floor has to leave it, and why figure 16 goes looking for the wind on a map of a continent rather than a country.
Figure 6 — the constraint binds in surplus, not in scarcity
Britain threw away 10.2 TWh of available wind in 2025, 13% of everything the transmission-connected fleet could have produced. That sounds like it should make the firmness problem worse. It does not, and the reason is the shape of when: curtailment correlates +0.75 with fleet output and −0.40 with residual demand. Below 10% of peak output the curtailment rate is 0.3%; above 80% it is 24%.
Against system tightness the same point from the other side. The tightest tenth of half hours carries 3.4% of the year's curtailment, the tightest hundredth 0.1%, and in the single tightest half hour of 2025 — 8 January, 44.4 GW of residual demand — just 0.22 GW was bid off.
It is not never, though. 147 hours of 2025 had both top-decile tightness and more than a gigawatt curtailed, the worst 4.3 GW on 3 February. That tail is the thing that grows if Scottish wind is built out ahead of the boundary. And it is a boundary problem, not a fleet-wide one: the top ten units carry 52% of all curtailment and the top twenty 74%, with Seagreen and Moray East and West the worst six — all north of B6.
Figure 7 — even at CP2030 capacity, these weeks need 37–45 GW from somewhere else
Ranking Dunkelflaute by lowest output finds the wrong weeks. The quietest seven days of these three years were in October 2025 — 3.3 GW of wind and solar — but demand was a mild 32 GW, so the gap everything else had to fill was 29.0 GW, barely above the ordinary 20.9 GW. Rank instead on the gap itself and every one of the four worst weeks lands in the heating season, the worst being 16–23 January 2025: 5.7 GW of wind and solar against 38.4 GW of demand, a 32.7 GW gap held for seven days, peaking at 43.5 GW. The failure mode is not a dip, it is a plateau.
The temperatures confirm it. All four weeks run 11–14 heating degree days a day against a three-year average of 4.9 — colder than 91–97% of all weeks. The October week with the lowest output sits at 4.1, dead average. Low wind is common; low wind in a cold snap is what costs money.
Scaling the same weather to Clean Power 2030 capacity — wind ×2.3, solar ×2.2 — lifts that week from 5.7 to 13.1 GW, while CP2030's demand growth lifts the other line from 38.4 to 41.2 GW. The average gap narrows from 32.7 to 28.0 GW, and a doubled fleet still covers only 32% of that week's demand against 15% today.
The worst half hour barely moves at all, and that is the number capacity adequacy actually turns on:
21–28 Jan 2023 — peak gap today: 40.7 GW · peak gap at CP2030: 37.7 GW
27 Feb – 6 Mar 2023 — peak gap today: 37.7 GW · peak gap at CP2030: 36.9 GW
26 Nov – 3 Dec 2023 — peak gap today: 44.2 GW · peak gap at CP2030: 45.3 GW
16–23 Jan 2025 — peak gap today: 43.5 GW · peak gap at CP2030: 45.1 GW
In two of the four weeks the peak gap gets worse. Doubling a fleet that is producing almost nothing still produces almost nothing, while the demand uplift is there regardless. Seven days is still seven days: what closes the remainder is storage, interconnection and flexibility, none of which is netted off above.
Figure 16 — the interconnection half of that sentence
Every proposed answer to a British wind drought is a line on a map to somewhere else: the Norwegian interconnectors, the North Sea grid, the Baltic, Xlinks' 3,800 km cable from Morocco. So it is worth asking the same question of a box running from the Sahara to the Barents Sea that figure 14 asked of Britain's own — where was the wind, in the weeks Britain ran short?
Britain averaged 7.34 m/s across those four weeks against 11.16 normally — 66% of its own average. All nine regions, with what each was doing relative to its own normal, and how far it is from the middle of Britain:
Norwegian Sea — mean: 12.01 m/s · of its own normal: 102% · distance: 1,326 km
The Faroes and the Iceland gap — mean: 11.29 · of its own normal: 83% · distance: 1,026 km
Iceland — mean: 9.50 · of its own normal: 92% · distance: 1,480 km
Gulf of Lion to Sardinia — mean: 8.45 · of its own normal: 107% · distance: 1,627 km
The Baltic — mean: 7.81 · of its own normal: 95% · distance: 1,476 km
Bay of Biscay — mean: 7.60 · of its own normal: 73% · distance: 962 km
The eastern North Sea — mean: 7.54 · of its own normal: 67% · distance: 598 km
Great Britain — mean: 7.34 · of its own normal: 66% · distance: —
Morocco (Xlinks) — mean: 6.24 · of its own normal: 94% · distance: 2,844 km
Iberia — mean: 5.22 · of its own normal: 87% · distance: 1,628 km
Seven of the nine beat Britain, so the first thing to say is that the wind was not switched off across a continent. But read the middle column. The nearest ground that beat Britain was having almost exactly Britain's bad week — the eastern North Sea at 67% of its own normal against Britain's 66%, 598 km away, which is to say inside the same weather system. Biscay, the next nearest, was at 73%. Everything that was genuinely windy for itself was 1,000 km or more distant: the Norwegian Sea at 102% and the Gulf of Lion at 107% were the only two regions of the nine running above their own average, and they sit at opposite ends of the map, 1,326 and 1,627 km away in almost opposite directions.
Morocco sits second from bottom there, though judging Xlinks by the wind column misses that it was mostly a solar project — 247 W/m² against Britain's 59 over those same weeks, and still at 381 W/m² at 5pm when Britain's residual demand peaks — and in any case DESNZ turned it down in June 2025.
So the escape from a British wind drought exists, and it is a long way off in two different directions. That is a statement about how much cable, not about whether the wind is there.
Worth noting what CP2030's extra demand actually is. Total GB demand grows 263 → 287 TWh, +11%, but residential demand falls: around 20 TWh of lighting and appliance efficiency more than outweighs the heat pumps added, since the plan only reaches the 600,000-installations-a-year rate by 2028. The growth is data centres, from 5 to 22 TWh, and hydrogen electrolysis at 11 TWh. On CP2030's own numbers the thing driving electricity demand growth this decade is not heat or transport — it is AI and hydrogen.
What this measures, and what it does not
Every gap in this piece is a gap against demand as it is now. The four worst weeks were scored against a Britain that still heats itself by burning things, in buildings the electricity grid has never had to warm. That is the quiet assumption underneath the whole exercise, and it is the one about to stop being true.
And it measures the problem, not the answer. There is no storage in any of these numbers, no demand-side flexibility, no interconnector flows — a wind farm at 4% of its peak reads as 4%, however full the batteries happen to be. That is the right way round for a first post, because every one of those things is bought in proportion to the hole it fills, and the hole is what figure 1 through figure 16 have been sizing. But it does mean nothing here shows that storage and flexibility cannot do the job. It shows what they would have to do: carry a fleet that spends the worst week of the year at 29% of its own average, in the season when demand is highest, with no help available from anywhere inside the island. Later parts put numbers on that — what within-day flexibility is worth on its own (about 11 GW of the apparent problem, it turns out), how much seasonal storage a system like this needs, and what it costs to keep the plant that covers the rest.
It matters because of what a data centre is and a house is not. A data centre is 22 TWh arriving as a flat band — more energy, same shape, and a shape this system is already built for. Heat is not flat. It is the most seasonal thing Britain does, it is concentrated into the same months as the calms, and it is currently carried by a network almost nobody thinks about: on a January morning the gas grid delivers close to three times what the electricity grid has ever delivered at its own all-time peak.
That network is scheduled to be dismantled and its job handed to the wires measured above — in exactly the weeks measured above. So the question the next part asks is not whether heat can be electrified. It is what happens to the shape of demand when it is, and whether the thing doing the heating still works when it is coldest.
Next, part 2: The Heat Grid Is the Big One — why the network being retired is larger than the one taking over, and what a heat pump actually manages on the coldest morning of the year.