NESO forecasts adequate winter margins, possible increase to system notices

NESO is forecasting a de-rated margin of 5.9 GW, an increase of 0.4 GW from its Early Outlook. According to NESO this is ‘adequate’ and remains comparable to recent winter periods. Its operational forecast remains sufficient, meaning that demand is expected to be met throughout the winter. Tight days are most likely to occur between mid-December and early February. NESO anticipates that changes to how it uses its operational tools could increase the number of system notices issued this winter. Great Britain (GB) is expected to be a net importer of electricity over the winter period.

De-rated margin

NESO continues to predict that there will be enough winter capacity to meet demand, and the reserve requirements for the period. Its forecast margin is 5.9 GW (9.5% of the Average Cold Spell peak demand), which is lower than last year’s margin of 6.1 GW but in line with recent winter periods. NESO’s predicted Loss of Load Expectation is 0.1 hours/year, which is within the reliability standard. NESO forecasts an increase in peak demand, offset by an increase in generation and storage capacity. NESO’s updated margin forecast is 0.4 GW higher than the early outlook’s prediction due to an updated methodology which has moderately increased the assumed Equivalent Firm Capacity (EFC). This is the amount of reliable baseload that could be replaced by another technology while maintaining the same level of reliability.

Forecasted De-rated margin for winter 2026/27

Source: NESO

Operational surplus

NESO’s forecasts still predict that the operational surplus will be sufficient throughout the winter period. However, NESO also predicts an increase in the frequency of the use of standard operational tools such as issuing system notices, although this will depend on developments. NESO notes that the ongoing El Niño event in the Pacific Ocean could lead to milder and wetter conditions throughout the period, with an increased chance of colder conditions in late winter. The operational surplus forecast has not been weighted to reflect the Met Office’s forecast for this winter as NESO has stated its aim is to take a prudent approach, considering the full range of possible weather conditions that could occur using data dating back to 1987. NESO will continue to incorporate the Met Office’s latest forecasts into its own forecasts.

Forecasted daily operational surplus for winter 2026/27

Source: NESO

Demand and supply

NESO expects daily peak demand to be marginally higher than last winter. The highest daily demand periods are expected to be from mid-December to early February. Peak winter demand has historically been within this period, though never on a weekend or over the Christmas fortnight. The lowest levels of generation are expected in November when outages are planned. Generator availability is expected to be higher than last winter due to the growth in generation capacity and the reduction in planned outages for the period. Actual availability will likely differ due to unplanned outages and there are expected to be tight days, including January and early February.

A contributing factor over this period is scheduled maintenance at Heysham, Hartlepool, and Torness nuclear power stations. These assets, which are all in excess of 600MW capacity, will be unavailable for between 2 weeks and over a month from 8 January to 18 February.

International markets

NESO is forecasting that imports through interconnectors will be available when required throughout the winter period, and that GB will be a net importer of electricity throughout the upcoming winter period. Wholesale prices in GB are expected to be higher than in France, resulting in net imports from France over the winter. Interconnector flows are expected to be more varied with Belgium and the Netherlands due to prices being more closely aligned. GB is also expected to be a net exporter to Ireland and Northern Ireland. French nuclear availability is expected to remain high throughout the winter period. Norwegian hydroelectric reservoir levels are lower than the 10-year minimum which could lead to more variability in flows through the North Sea Link than last winter.

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