The heatwave and the data centre want the same thing, and Britain can't give it to both.

Britain's electricity grid doesn't store much. It is, second by second, a balancing act: the system operator must match the power flowing in from generators to the power being pulled out by everything plugged in, holding the whole network at almost exactly 50 hertz. Too much demand, too little supply, and the frequency sags; the operator's job is to never let the two drift apart. Most of the time you never notice it happening.

This week it will be noticed. The red extreme-heat warning the Met Office issued on Monday — 38°C forecast for Wednesday and Thursday — is, among other things, an electricity event. As the temperature climbs, millions of fans, fridges working harder and air-conditioning units switch on across the south-east at once, and demand spikes precisely when the heat is making the grid's own equipment less efficient. The Met Office said as much in plain terms: the heatwave will bring impacts "to power and water supplies." A hot Wednesday is a stress test for the system.

Now hold that picture next to the other thing Britain is trying to plug in. The government wants a wave of AI data centres — its "AI Growth Zones" — and a single large one can draw as much electricity as 100,000 homes. The crucial difference is in the shape of the demand. A heatwave is a spike: it comes, it peaks, it passes. A data centre is the opposite — a flat, relentless, always-on draw that never dips, day or night, summer or winter. The grid can plan around a spike. A permanent new block of gigawatt-scale demand is a different kind of problem, and it's why connection queues have grown so long that some projects have been told to wait up to fifteen years.

There's a second collision, and it's the one people forget: water. Many data centres are cooled with it, evaporating thousands of litres to shed the heat their chips produce — and they need it most on exactly the days a heatwave has the rest of the country worrying about reservoirs and hosepipe bans. The machine that runs without people still drinks like a small town.

This is the brittleness underneath the ambition. None of it means the data centres shouldn't be built; the same compute may one day run the diagnostics, the forecasts, even the grid itself. But you can't wish the electrons or the water into being. West London has already seen the trade-off made concrete: parts of its grid hit capacity in 2022, and new housing was held up because there wasn't power for both. That's the choice in miniature — homes or machines, on a network that, this Wednesday, will be straining to power what we've already got.

Why it matters here — The heat will pass by the weekend. The data centres won't. Britain is committing to a permanent new demand on a grid that already can't always keep up, and the bill — in connection delays, in water, and eventually in the price on the household account — lands here, on the same network keeping homes cool in a heatwave.