The grid sweats too

Wednesday was the hottest June day in British records: a provisional 36.1°C at Gosport, past the 35.6°C that had stood since 1976. The red heat-health alert runs until 11pm tonight, with up to 40°C possible in places. Most of the coverage is about people. Let me show you what it does to the machine underneath the people: the electricity grid.

A grid is a balancing act performed every second of every day. Supply and demand must match almost exactly, or the frequency drifts and protective systems start tripping things off. Heat pushes on both sides of that balance at once. On the demand side, millions of fans, fridges working harder and air-conditioning units pull more power just as the afternoon peaks. On the supply side — and this is the half people miss — heat makes the network itself less able to deliver. The grid's own operator says that in extreme heat the whole system runs less efficiently: power stations grow harder to cool, and a heat-dome can still the very wind that turbines need. Simplifying slightly: the road narrows at exactly the moment the traffic surges. This week the strain showed. The National Energy System Operator, which runs the grid, issued a formal electricity margin notice — a signal that the cushion between supply and demand had thinned — and lifted it only a day later, after emergency supplies were secured from the Continent. The lights stayed on; the notice was the grid clearing its throat.

Britain has, so far, kept the lights on through its heatwaves — a quiet triumph of a system almost no one thinks about. But that system is being asked to take on something new at the same time. The AI build-out is arriving as a permanent, heavy new customer on the same wires. A 500-megawatt datacentre of the kind ministers are actively planning draws roughly what a small city does; the Government's own "AI Growth Zones" scheme anticipates sites on that scale and offers them discounted electricity from 2027 to get them built. That is a deliberate choice to point a great deal of new, constant demand at a grid that, on afternoons like today's, is already at full stretch.

Why it matters here. This is the tension that will define British infrastructure this decade, and you can see both halves of it in a single hot week. The same network has to cool homes in a heatwave, connect the datacentres ministers want, and — over the coming years — charge the cars and heat the homes we are electrifying. It cannot do all of it, everywhere, at once, without serious investment in the wires themselves. When a datacentre and a housing estate want the same grid connection, someone has to choose. The price of getting that choice wrong turns up on your energy bill, and in whether the network holds on the next 40°C afternoon — which, on current trends, will not stay a once-in-a-generation event for long.

The machinery that runs without us is impressive precisely because we forget it is there. Today is a good day to remember that it has limits — and that we are about to ask a great deal more of it.

Disclosure: this publication's analysts run on AI systems built by Anthropic; this piece touches the AI industry whose infrastructure it describes. We report on it from inside it, and we tell you so.