Demand

The shape of the day

Demand is lowest in the small hours and rises through the morning as the country gets up. In winter the day’s high point is usually the early evening, when lighting, heating and cooking overlap with people arriving home; in summer the day is flatter and often peaks mid-morning, because the evening is warm and light.

⚠ The line is not total electricity use. It is the demand the transmission system met. On a sunny afternoon rooftop solar can supply several gigawatts that never touch the transmission system, so national demand falls while people are using more electricity, not less. The midday dip is a measurement boundary, not a change in behaviour. The third of “The three demand figures” above, underlying demand, adds that estimate back.

The forecast, and its limits

The dashed line is NESO’s national demand forecast. It forecasts the same quantity the solid line measures — national demand, on the transmission boundary — so the two can be compared directly, which is not true of every forecast published against every outturn.

How close it has been is the sentence under the Demand today chart: the mean absolute error of the last day-ahead publication against every half hour of outturn today. Each miss counts at its size whichever way it went, and the direction is given separately — a forecast a gigawatt out in alternating directions averages to nearly zero if the signs are kept. It is compared with national demand, never with the transmission figure, which would show a two-gigawatt “error” that is only a difference of boundary.

Demand margin — the forecast surplus of available generation over demand at each day’s peak, two to fourteen days ahead — is a question about supply. It is on the System page, with the fortnight it covers.

Temperature, and the limits of the relationship

Cold weather raises demand, and by a lot: a few degrees below the seasonal normal is worth gigawatts across a GB winter evening. The Demand and temperature panel opens with Elexon’s daily GB average against the seasonal reference, which is why it gives the difference and not just the bare number — 8°C means very different things in January and June.

⚠ Temperature does not determine demand. Day of the week, daylight, holidays and what is on television all move it, and the link between degrees and megawatts is not fixed — it depends on how much of the heating stock is electric, which changes year on year. Treat it as one strong influence among several, not as a formula.

How figures are marked. A figure with no mark is measured; an estimate is hatched; a forecast is dashed. A total or a share with the solar estimate inside it says so: incl. est. solar.

More in the methodology: supply and demand never quite match →