What Today's Electricity Margin Notice Tells Us About the Future Value of Flexible Capacity

NESO (National Electricity System Operator) issued the fifth EMN (Electricity Margin Notice) of 2026 overnight into Monday 28 September, for the 16:00 to 19:00 evening peak.

Today's EMN was ultimately stood down and posed no risk to security of supply. However, it reinforces the growing value of flexible capacity. As the electricity system becomes increasingly weather-dependent, assets that can reduce demand or provide capacity at short notice will play an increasingly important role, while generating additional revenue streams for participating businesses.

Brook Green Supply gives customers access to the Capacity Market and to GB's flexibility markets, so we closely monitor Electricity Margin Notices and what they mean for asset owners, energy users and the wider power system.

Today’s Events

The notice initially indicated a 1,400MW shortfall against a 700MW contingency requirement. A further 1,900MW of generation was excluded from the operational margin due to transmission constraints. NESO called the notice routine and precautionary and said supply to customers was not at risk. 

The demand backdrop was ordinary. The issue was driven largely by the supply side, with wind generation forecast to fall to around 2.5GW across Great Britain during the evening peak. NESO expected the shortfall to ease overnight as output recovered. The four EMNs before today – on 24 June, 26 June, 9 July, and 12 August – were each stood down without escalating.

Market prices pointed to a tight but manageable system rather than one under significant stress. The N2EX day-ahead auction reached £268/MWh at 18:00 and averaged £238/MWh across the notice window. Tuesday 22 September delivery peaked higher, at £356/MWh, with no EMN. By late morning today, NESO had revised its numbers; the shortfall was 104MW, down from 1,400MW.

Impact on the Capacity Market 

No Capacity Market Notice was issued. Capacity Market Notices follow a separate process and are typically associated with available capacity coming within around 500MW of demand plus the required operating margin. The EMN notice was cancelled at 15:00, an hour before the window opened, with no emergency Capacity Market action taken. Capacity Market participants maintained their operational procedures throughout the day and continued to earn CM revenues as normal.

Looking Forward

The more important story is not today's notice itself, but what it says about the changing nature of system tightness. EMNs have historically been associated with winter peak demand. However, all five notices issued so far in 2026 have occurred between late June and late September, highlighting the growing challenge of managing evening demand as solar generation fades and renewable output fluctuates.

The conclusion is clear: taking part in the Capacity Market and the flexibility markets pays, and it matters. For the consumer, it turns availability that would otherwise sit idle into revenue, whether that is contracted capacity payments agreed years ahead or a within-day turn-down call. For the system operator, every additional registered, prequalified asset deepens the pool of offers NESO can call upon before it needs an emergency tool, at a price set by competition rather than by instruction. 

How to optimise your sites in a volatile market

Participating in the Capacity Market and the flexibility markets together enables a site to earn from both, while supporting system stability at every stage. Contracted Capacity Market payments underpin the margin years ahead, and short-term flexibility covers what remains when a notice is issued.

Brook Green Supply's tailored, end-to-end market access products take your asset from registration through to offering and settlement, built around how it actually connects and responds. Get in touch today to find out how much value your assets could be earning.