System
Checks on this page
Each feed is judged by the age of its newest data against its own timetable: “late” means the last check found nothing newer. Settlement period now: SP–.
Supply must match demand every second on GB’s grid, and frequency shows how well it is doing. This page tracks it on three timescales — real time, forecast and settled — and why they can disagree harmlessly.
System frequency
FREQ · 15-secondFrequency against its limits
The last hour, against the limits that matterSpare capacity, the fortnight ahead
Forecast surplus of available generation over demand at each day’s peakImbalance price
What being out of balance cost, over the last 24 hoursMarket price
What electricity traded for on the exchanges, volume-weighted across them, over the settlement periods that tradedAbout these figures
Reading the frequency · three readings of one thing · two prices
Reading the frequency
The grid runs at 50 Hz, and the frequency is a direct physical readout of whether generation and demand match right now. More generation than demand and it rises; more demand than generation and it falls. Nobody takes and publishes it on a schedule: it is what the spinning machinery is doing, sampled every 15 seconds.
⚠ Small deviations are normal and constant. The system is never exactly balanced; the balancing mechanism pulls it back towards 50 Hz all the time. What matters is how far it moves and for how long, which the panel above measures against the two limits that mean something.
Three readings of one thing, and none of them causes the others
Indicated imbalance, net imbalance volume and frequency all describe whether GB is short of generation or has a surplus. They are not cause and effect. One is NESO’s forecast in megawatts, one is a settled volume in megawatt-hours for a period that has closed, and one is a physical reading taken every quarter of a minute.
They often point the same way. When they do not, the usual reason is that they describe different windows, not that one of them is wrong. The imbalance does not set the frequency: it is one input among many, acted on continuously, which is the whole purpose of the balancing mechanism.
Two prices, two questions
The market price is what electricity traded for on the exchanges, volume-weighted across them. The imbalance price is what parties pay for being out of balance — for using more or less than they contracted for. They answer different questions, and they routinely differ by a lot.
A negative imbalance price is ordinary on a windy night and is not an error: the system was long, and it was worth paying people to take electricity away.
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: system frequency, and two limits that are not the same →