The UK’s shaky ‘firm’ capacity

As many readers know, the GB electricity grid narrowly missed a blackout at the start of January. As was widely reported that the time, it looks as though we were down to our last half a gigawatt of spare capacity. After that, the only option available to grid managers is to switch people off, either through voluntary arrangements (so-called ‘demand-side response’) or through compulsion – rolling blackouts, in other words.

It is beyond doubt, therefore, that the system is very fragile. When all is said and done, the blame can be laid squarely at the feet of renewables, our 45 gigawatts of which delivered just 2.6 gigawatts of actual output during the evening peak of 8 January 2025. In some ways, we got lucky – at other times this year, renewables output has fallen to 0.5 gigawatts. If that had happened on 8 January, grid managers would simply not have been able to find the extra capacity required.

In the event, we had just enough reliable generation – gas and nuclear – to fill the gap. They ran pretty much flat out, and the lights stayed on.

However, there’s a problem, and it’s a big one.

Very few gas-fired power stations stay open for longer than three decades, and much of the GB fleet is now very long in the tooth. The Medway power station will hit age 30 this year, and Little Barford, Connah’s Quay and Keadby will follow in 2026. To make things even worse, the Hartlepool nuclear power station is scheduled to close next year too, while Heysham will close in 2027.

At the moment, the grid is making reassuring noises. The Capacity Market, which subsidises power stations to “be there” when renewables decide not to deliver, has booked “firm” capacity of 43 gigawatts for winter 2027–28, and they will expect to top that up nearer the time.

But look a little closer, and the picture becomes less rosy. For example, more than 1 gigawatt of the total is batteries, which will probably only be able to deliver for two hours. By counting them as firm capacity, grid managers are essentially crossing their fingers and hoping that stress events are very brief. Another gigawatt is “unproven” demand-side response – people who say they are willing to switch off when supplies are tight. Will they do so in practice? Nobody can be sure.

Nearly 7 gigawatts of the total is interconnectors, the “firm” nature of which is increasingly open to question. As we know from previous grid crises, when there is a shortage of electricity here in the UK, there is usually a shortage across Western Europe, because wind speeds are highly correlated. The result is a bidding war for the limited supplies of electricity. Those with surplus firm power – notably Norway and France – are increasingly asking why their consumers should have to put up with sky-high prices because the rest of Europe runs its electricity system in a completely irresponsible way. Oslo in particular seems likely to cut the interconnectors in future, rather than endure the economic pain.

But the bulk of the UK’s firm capacity is gas-fired, and much of this is shaky, rather than firm. Nearly 2 gigawatts of the total is represented by a proposed new unit at Eggborough. However, a few weeks ago, the developers relinquished the capacity contract for one of the two units and, with a building contractor apparently yet to be appointed, it remains unclear whether the other will go ahead either.

Also in the gas-fired segment are all those decrepit units mentioned above. Their advanced ages means that their status as firm capacity is open to question. It’s all very well a rusting old power station saying it will be there to help out in 2027, but when the engineers try to switch it on, will it actually deliver? There are fines for failing to do so, but not so onerous as to discourage operators from taking the subsidy and crossing their fingers.

On the face of it then, it is quite possible that a large chunk of our 43 gigawatts of firm capacity for winter 2027-28 may not materialise. There are a few scraps of hope around. EDF have apparently indicated that the lives of Hartlepool and Heysham might be extended, which would certainly help things. It is possible that some of the older gas-fired units might take new capacity market contracts to convert to single-cycle power stations (OCGTs)  – more suited to running at low capacity factors. But OCGTs deliver much more costly power than CCGTs, so hard-pressed consumers should expect even more pain. Finally, grid managers will be hoping that electricity demand doesn’t rise as quickly as forecast over the next few years. That might give them a little more breathing space.

But a mixture of quick fixes and Hail Marys only to adds to the already worrying impression of a grid that resembles (to misquote the eighteenth century diplomat George MacCartney) “an old, crazy, first-rate Man of War, which a fortunate succession of able and vigilant officers have contrived to keep afloat for these thirty years past”.

How much longer it can be kept from sinking remains to be seen.