What is demand, exactly?

Britain publishes several numbers for how much electricity it is using. None of them measures quite what you would think, and a growing share of what they describe is no longer measured at all.

There is a number on a lot of dashboards that says how much electricity Great Britain is using right now. It looks like the sort of thing you would measure with a very large meter.

It isn't. There is no such meter, there never was, and the further you look into how the number is actually assembled the less it resembles a measurement at all. Some of it is metered. Some of it is inferred. Some of it is a parameter somebody set once. And a growing share of the electricity Britain generates is now invisible to it by construction — not because nobody has got round to metering it, but because the way the system is defined means the quantity is never formed in the first place.

This is not a scandal. Every one of these choices was reasonable when it was made. It is more interesting than a scandal: it is what happens when a measurement framework built for about a hundred large machines is asked to describe a system with two million.

It is also, I think, the reason for a question I get asked more than any other, usually in some version of: how good are we actually getting at forecasting wind and solar? There is a fair amount of heat in that question at the moment.

My honest answer is that the forecasting is better than most people assume and the measurement is worse — and that the two are joined at the hip, because a forecast is only ever as good as the outturn you score it against. Here is the tell, which gets a section of its own further down. Britain's day-ahead wind forecast runs about 14% high, on five days out of six. That sounds like a forecasting failure. It is not one. The forecast is answering a question nobody meant to ask it, and working out which question takes you directly into how demand is defined — which is where the rest of this piece goes.

So this is not really an article about forecasting. It is an article about what a forecast is being compared against, and how much of that comparison is estimated rather than measured.

The old world, where this all made sense

Start where the definitions started. In the 1990s, almost everything that generated electricity in Britain was a large machine bolted to the transmission system: coal stations, a handful of nuclear plants, the first big CCGTs. A few dozen sites, all of them individually metered, all of that metering read centrally and in real time by the system operator.