Electricity Margin Notices, Explained

"National Grid Issues Urgent Warning"

Every winter, and occasionally in other seasons, a familiar headline cycle plays out. The National Energy System Operator (NESO) issues an Electricity Margin Notice (EMN), newspapers report that the grid is "on the brink", and a day later the notice is quietly cancelled and nothing happens.

Both halves of that cycle are real. The notice is genuine — it means the safety cushion for a coming period looks thinner than NESO wants. And the anticlimax is genuine too — because an EMN is designed to produce that anticlimax. It is a request for more capacity, made early, through an official channel, so that the market can close the gap before it matters.

This post unpacks what an EMN actually is, the number that triggers it, what happens after one is issued, and how you can watch the underlying margin on GridHz.

The number behind the notice: de-rated margin

NESO continuously forecasts the operating margin: how much generation and import capacity will be available above expected demand, for every half-hour settlement period, days in advance.

Crucially, the margin is de-rated. A gas plant that is technically available might still trip on the day; a wind forecast might not materialise. So each source is discounted by its expected reliability — 2,000 MW of "available" plant might count as, say, 1,800 MW of de-rated capacity. The result is the de-rated margin: the buffer NESO can actually count on.

Alongside it, NESO publishes the Loss of Load Probability (LOLP) — the modelled probability that supply falls short of demand in a given half-hour. On a normal day the LOLP is effectively zero for every period. It only lifts off the floor when the de-rated margin gets genuinely thin.

Both figures are published through Elexon per settlement period and refined at 12, 8, 4, 2, and 1 hours ahead of each period — so the market watches the margin converge as real time approaches.

What triggers an EMN

NESO holds a required contingency — a minimum margin it wants in hand to cover plant failures and forecast errors. The requirement varies with system conditions; it is not a single fixed number.

When the forecast de-rated margin for a coming period drops below that requirement, NESO issues an Electricity Margin Notice through the official system-warning channel (published to the whole market via Elexon's BMRS). The notice states the period affected and the size of the shortfall — for example, "a forecast shortfall of 800 MW of the margin required for the period 16:30 to 20:30".

Two things an EMN is not:

• It is not a prediction of disconnections. The shortfall is against NESO's desired cushion, not against demand itself. Even at the moment an EMN is issued, forecast supply still exceeds forecast demand — the concern is that the buffer above demand is smaller than NESO wants.
• It is not the last line of defence. Beyond an EMN sits a ladder of escalation — further notices, maximum-generation instructions, voltage reduction, and only in extremis controlled demand disconnection. EMNs are the bottom rung, and the vast majority of tight periods never climb past it.

What happens next

An EMN changes behaviour, which is the whole point:

• Generators bring plant back from outages early, delay maintenance, or make part-loaded units available at full output.
• Interconnectors respond to the price signal — a tight GB system usually means high GB prices, which pulls imports from France, Norway, Belgium, the Netherlands, and Denmark.
• Flexible demand — from industrial users to aggregated households in the Demand Flexibility Service — reduces consumption during the flagged window, often for payment.
• The Balancing Mechanism gives NESO direct control to accept offers for more output in real time.

In most cases, within hours the margin recovers and NESO publishes a cancellation. That is not a false alarm; it is the mechanism working exactly as designed. A market that only found out about tightness in real time would be far more dangerous than one warned six hours ahead.

Why EMNs cluster on cold, still evenings

The tightest periods in GB are almost always winter weekday evenings between roughly 16:30 and 19:30: demand peaks as lighting, heating, and cooking loads stack on top of industrial demand, solar output is zero, and if a stationary high-pressure system has becalmed the wind fleet at the same time, the de-rated margin can compress quickly.

These are the same conditions this dashboard's other panels make visible: low wind share in the generation mix, high demand, and heavy reliance on gas and imports. Margin tightness is not an independent phenomenon — it is the adequacy dimension of the same weather-driven system state that moves inertia and prices.

(A subtlety worth knowing: the stability worst case is nearly the opposite — mild, windy, low-demand nights, when inertia bottoms out. Tight margin and low inertia are both "system stress", but they happen in different weather.)

A short history

The EMN replaced the older Notification of Inadequate System Margin (NISM) in 2016, partly because "inadequate" invited exactly the wrong reading. Related notices you may also see:

• Capacity Market Notice (CMN) — an automatic notice, triggered four hours ahead when the forecast margin falls below a set threshold. It exists to put Capacity Market contract holders on alert and is even more routine than an EMN (it fires mechanically, with no operator judgement involved).
• Demand control notifications — much rarer, and the point where a tight margin becomes an operational event rather than a market signal.

Actual demand disconnection due to inadequate margin is extraordinarily rare in GB — the events people remember (like August 2019) were sudden plant failures and frequency events, not adequacy shortfalls that EMNs address.

Watching the margin on GridHz

GridHz now surfaces both halves of this story on the dashboard:

• The System Margin panel (Stability tab) charts NESO's de-rated margin for the past 24 hours and the next ~12, with the loss-of-load probability alongside. On a comfortable summer day you'll see margins of 8–10 GW; a tight winter evening can compress that below 2 GW.
• The System Warnings panel (Market tab) lists official margin-relevant notices — EMNs, Capacity Market Notices, demand-control messages — from the last 7 days, with their published shortfall. Active notices also appear in the alerts bar.

So next time a headline announces that the grid has issued an "urgent warning", you can check the actual number it refers to, watch the market respond, and — most likely — watch the notice get cancelled.

Data: NESO margin forecasts (LOLPDRM) and official system warnings, published via Elexon BMRS. GridHz refreshes both every few minutes.