The NESO Winter and Summer Outlooks Explained

Twice a year, before the cold months and before the warm ones, Britain's electricity system operator publishes a forecast of whether the grid should hold up over the season ahead. The National Energy System Operator, NESO, is the body that does this work. It was established in October 2024 as the whole system planner, the operator of the electricity system and expert advisor to government and Ofgem¹. Its seasonal outlooks are the documents that translate a mass of engineering and market data into a single question a householder recognises: will there be enough electricity this winter?

The headline answer for winter 2025/26, from the government's statutory security of supply report, was a de-rated margin of 6.1GW, equal to 10.0% of the Average Cold Spell peak demand for that winter, with a loss of load expectation below 0.1 hours per year². In plain terms, the system was expected to carry a cushion of spare capacity equal to about a tenth of the coldest-weather peak, and the residual risk of demand going unserved was assessed as very small. That is the kind of figure the Winter Outlook exists to produce and defend.

This page explains what the outlooks are, how the numbers are built, and what they do and do not tell you about the risk of shortages in your home. It also covers the Summer Outlook, which answers a different set of questions, and how to read the published reports without reading more into them than they can bear.

The seasonal Outlooks explained, and their limits

The Winter and Summer Outlooks are forecasts, not policies and not promises. They are NESO's best engineering view of how much electricity Britain will be able to generate and import in the season ahead, set against how much it expects to consume. They feed the government's formal monitoring of security of supply, which is where the published margin figures for winter 2025/26 come from². But a forecast of adequacy is a statement about probability, not a guarantee of outcomes, and it is worth being clear about the distinction before reading any of the numbers.

A useful comparison comes from a different government document, the Net Zero Technology Outlook, which carries the explicit caveat that "The Outlook is not a statement of government policy."⁴ The same discipline applies to seasonal outlooks: they describe an expected position given current plant, current market arrangements and stated assumptions, and they change when those inputs change. A generator breaking down, an interconnector exporting instead of importing, or a colder-than-average spell all move the real position away from the forecast.

The outlooks also sit inside a wider family of NESO publications. NESO's whole-system advisory role includes producing a Strategic Spatial Energy Plan, which sets out future generation and storage needs, and assessing the resilience of the energy system in Great Britain⁵. The seasonal outlooks are the short-horizon end of that work: where the Strategic Spatial Energy Plan looks years ahead, the Winter Outlook looks months ahead. Both are advisory in character, and neither substitutes for the real-time balancing that keeps the grid stable, which is covered in balancing services and frequency response.

For households, the key point is that an outlook is a risk assessment written by the body that runs the system. It is not a warning of imminent cuts, and it is not a marketing document. When the margin it reports is healthy, that is a genuine statement of confidence; when it is thin, that is a genuine statement of concern, and it usually triggers action long before any household notices anything.

Why we publish before every season

The reason is timing. Electricity cannot be stored in the wires, and the tools that improve a season's position, from contracting reserve capacity to arranging flexibility, have to be lined up before the season starts, not during it. The Capacity Market, which pays providers to be available in a given delivery year, holds its auctions well in advance: the T-1 auction for the 2025/2026 delivery year concluded on 5 March 2025, the T-4 auction for 2028/2029 concluded on 11 March 2025, and the next auctions were scheduled for March 2026². The Winter Outlook tells NESO, government and the market whether that machinery has delivered enough, while there is still time to add to it.

The outlooks also serve the market itself. Generators, suppliers and flexible users make commercial decisions partly on the basis of expected tightness: when a winter looks tight, the value of being available rises, and that price signal draws in capacity and demand reduction. The Parliamentary Office of Science and Technology describes demand side response as covering "a range of services that vary the demand of both domestic and commercial consumers to help balance the power grid"⁶, and the outlooks are part of what tells those consumers when their response will be worth most.

There is a longer game as well. The government's clean flexibility roadmap records that NESO published a target to secure an additional 750MW of non-domestic flexibility in its services and markets by 2030³, and seasonal outlooks are one of the ways the need for that flexibility is evidenced season by season. Each report is both a snapshot and an argument: a snapshot of the coming season, and an argument about what the system needs more of.

Winter Outlook scope: adequacy of supply and the margin

The Winter Outlook answers one central question: can expected supply cover expected peak demand through the coldest spell of the winter? Its headline output is the margin, expressed both in gigawatts and as a percentage of peak demand. For winter 2025/26 the statutory assessment reported "a derated margin of 6.1GW (10.0%) of the Average Cold Spell (ACS) peak demand for this winter"². The Average Cold Spell is a defined weather condition: not the worst conceivable winter, but a demanding one that the planning standard expects the system to withstand.

The word "de-rated" matters. Raw nameplate capacity overstates what a system can rely on, because wind farms produce less in calm weather, thermal plants have outage rates, and interconnector flows depend on conditions abroad. De-rating discounts each source of supply to a reliability-adjusted figure before the margin is calculated. That is why a margin of 10.0% of peak demand represents a larger gross fleet: the calculation has already subtracted the expected unavailability.

Alongside the margin, the Winter Outlook reports the loss of load expectation, the formal statistical measure of how often demand might go unserved. For winter 2025/26 this was assessed at below 0.1 hours per year², a very small residual risk. This is the number that the security standard is actually written against, and it is the one to look at when headlines claim a winter is "tight": a thin margin raises the loss of load expectation, but the two are not the same thing.

The Winter Outlook also covers the tools available if the forecast position deteriorates: interconnector imports, reserve contracts, and demand side response. Demand side response exists, in the words of the Parliamentary Office of Science and Technology, "to manage peak demand to reduce the amount of network investment required to meet an increasing electricity demand, caused by the electrification of heating and transportation; and to match demand with the output of renewable electricity generation, which tends to be intermittent"⁶. How those tools work in practice is covered in electricity margins, shortfall notices and the risk of shortages.

Summer Outlook scope: low demand and outage planning

The Summer Outlook answers a different question. Summer is not when Britain's grid is most stressed by demand; it is when demand is lowest and when much of the generating fleet is taken offline for maintenance ahead of the next winter. The Summer Outlook's job is to check that the reduced fleet still covers the reduced demand, and that planned outages can be scheduled without leaving the system exposed during an unexpected summer peak or a low-wind period.

Summer brings its own weather assumptions. Building standards in Scotland define Design Summer Year 1 as "intended to represent a moderately warm summer intended to represent 2011 to 2040"⁷, and the same thinking about representative rather than extreme weather runs through system planning. A summer outlook has to consider both low demand and the possibility of unusually warm spells that raise cooling loads, alongside the maintenance schedule that deliberately removes capacity from service.

The summer report also matters for the winter that follows. Maintenance windows are when plants are serviced so they are available when needed most, and the Summer Outlook is where the sequencing of those outages is checked. A summer in which maintenance slips is a winter in which availability assumptions weaken, so the two outlooks are connected: the Summer Outlook partly protects the position that the next Winter Outlook will report.

For households, the Summer Outlook rarely makes news, and that is usually the point. It is the quieter of the two documents, but it is built on the same discipline of comparing reliable supply against expected demand, and it is the reason the system enters each winter with its plant serviced rather than patched up mid-season.

Building the assessment

The method behind both outlooks is a structured comparison: expected demand on one side, de-rated supply on the other, with assumptions stated explicitly. On the demand side, weather correction is central. Ofgem's review of typical domestic consumption values describes applying "the latest view of seasonal normal" to allow "the most up to date view of weather correction for the gas dataset"⁸, and the same principle of adjusting consumption to a representative weather year applies to electricity. Not every dataset is weather corrected in the same way: the National Energy Efficiency Data-Framework notes that "electricity consumption is less affected by the weather, so this data has not been weather corrected"⁹, which is a reminder that assumptions differ by fuel and by purpose.

On the supply side, the assessment counts the fleet that is expected to be available, discounts it for expected unavailability, and adds the expected contribution of interconnectors. The connections queue is part of this picture, because new capacity only counts once it is genuinely connected. The government's response to the electricity distribution networks study describes reforms that "will deprioritise almost 500GW of projects in the oversubscribed connections queue"¹, a measure of how far the queue overstates what will actually arrive. An outlook counts what is real, not what is queued, and the treatment of the queue is explained in the grid connection queue and connections reform.

The assessment also draws on the growing body of flexibility data. The clean flexibility roadmap records that NESO will publish skip rate figures for constrained low-carbon flexibility behind thermal constraints by Q3 2026, and that as of May 2026 skip rates "are consistently at a rate of between 8 and 12 percentage points lower than in 2025"³. Skip rates, the frequency with which flexible assets are curtailed, feed directly into how much flexible supply an outlook can reasonably assume.

Finally, the assessment is not a one-off. NESO is expected to establish regional strategic boards to steer and oversee the development of Regional Energy Strategic Plans¹, and its methodology for constrained low-carbon flexibility in those plans is due to be published in Summer 2026³. The seasonal outlooks sit at the sharp end of this planning stack: they are where the long-range assumptions meet the actual plant available this season.

The numbers: margin, loss of load and their basis

The two numbers to read first are the de-rated margin and the loss of load expectation. For winter 2025/26, the statutory security of supply report gives both: a de-rated margin of 6.1GW, or 10.0% of the Average Cold Spell peak demand, and a loss of load expectation below 0.1 hours per year². The margin tells you the size of the cushion; the loss of load expectation tells you the residual probability that the cushion fails.

Behind these figures sits the Capacity Market, the main mechanism that buys firm capacity years ahead. It is technology neutral², meaning any capable resource can compete, and its auctions are held on a fixed calendar: the T-1 auction for the 2025/2026 delivery year concluded on 5 March 2025, the T-4 auction for the 2028/2029 delivery year concluded on 11 March 2025, and the next auctions were scheduled for March 2026². The relationship between the outlook and the auction is explained in the Capacity Market.

Interconnector flows are the other figure worth watching. Britain trades electricity with its neighbours through subsea and overland links, and the direction of flow at any moment depends on relative prices and scarcity, not on British need alone. How those links work, and how much Britain can rely on them in a tight spell, is covered in electricity interconnectors. An outlook assumes a de-rated contribution from imports precisely because neighbours may need their own power in the same cold snap.

The last figure in the set is flexibility. NESO's target of an additional 750MW of non-domestic flexibility by 2030³ is the forward-looking number, but the seasonal outlooks are where the current contribution of flexible demand is counted against the current margin. Household flexibility, and what it does for the grid, is covered in household demand flexibility.

Influences: weather, imports and available plant

Three variables move a seasonal forecast more than any others: the weather, the direction of imports, and whether the plant fleet is actually available. Weather enters through the demand assumptions, which are corrected to a seasonal normal view⁸, and through the supply assumptions, because wind output in a calm, cold spell can fall far below average. The Climate Change Committee has warned that the UK is struggling to keep pace with climate change impacts, including "risks to supply of food, goods and vital services due to climate-related collapse of supply chains and distribution networks"¹⁰, and energy networks are part of that exposure. The Utility Regulator's price controls in Northern Ireland require business plan strategies to consider how climate change could impact networks and to outline adaptation actions to manage risks to services⁵, a sign that weather risk is now treated as a planning input, not a footnote.

Imports are the second variable. Interconnectors can flow either way, and in a European-wide cold spell the assumption that power will flow towards Britain weakens. That is why interconnector contributions are de-rated in the margin calculation rather than counted at full capacity. Plant availability is the third: the de-rating of thermal plant reflects expected outage rates, and the maintenance schedule agreed through the Summer Outlook determines how much of the fleet is serviced and ready. A single large plant failing during a peak does not invalidate the forecast, because the margin exists to absorb exactly that kind of event.

The longer-term pressure on all three variables comes from the changing shape of demand. The electrification of heating and transport is increasing electricity demand and making it more variable⁶, which raises the value of accurate seasonal forecasting. The government's Future Homes and Buildings Standards consultation lists as a policy priority to "reduce energy demand of homes and non-domestic buildings by requiring high performing building fabric and building services in new buildings, thereby improving energy security"¹¹, which is the demand-side counterpart to the supply-side machinery the outlooks describe.

What this means in practice is that the outlook's margin is not a single number that either holds or fails. It is the centre of a distribution of possible outcomes, shaped by weather, imports and outages, and the loss of load expectation is the summary of how much of that distribution spills into shortage.

The Outlooks and your home's energy security

The honest answer is: less than the headlines suggest, and more than nothing. The outlooks do not change anything in your home on the day they are published. They do not set your bill, and they do not trigger any action by your supplier. What they do is describe the system-level risk environment your home sits inside, and that environment does eventually reach households, through prices in the near term and through reliability in the extreme case.

On price, the connection is indirect but real. Tight margins raise wholesale electricity prices during scarcity periods, and wholesale costs feed into household bills over time. Conversely, the Parliamentary Office of Science and Technology notes that demand side response "could make household energy bills cheaper"⁶, and the outlooks are part of what justifies investment in that response. The mechanics of how wholesale prices reach your bill are covered in the wholesale gas and electricity markets.

On reliability, the outlooks are the early-warning layer of a long ladder of protections. Before any household loses supply, NESO would use emergency assistance from interconnectors, call on contracted reserve and flexibility, and reduce demand through commercial users. Rota disconnection sits at the far end of that ladder, and the loss of load expectation below 0.1 hours per year for winter 2025/26² is the measure of how far away that end is expected to be.

For a household thinking about its own position, the outlooks are one input among several. The bigger levers on household energy security are the ones a home controls itself: reducing demand through insulation and better fabric, which the National Energy Efficiency Data-Framework measures in terms of "the impact of installing home efficiency measures such as loft insulation on household energy consumption"¹², and which the Future Homes and Buildings Standards explicitly tie to improving energy security¹¹. These are covered in energy security and household independence.

Where you live also shapes what the outlook means for you. Northern Ireland's electricity system is part of the all-island Single Electricity Market, so the GB outlooks describe the system that serves Great Britain, not the whole UK. Householders in Northern Ireland can turn to NI Energy Advice, which "offers independent and impartial energy advice to domestic householders in Northern Ireland"¹³, plus referrals to energy grants and other sources of help. Scotland and Wales are served by the GB system the outlooks cover, though their generation mix and network arrangements differ, as described in energy supply in Scotland and energy supply in Wales.

Interpreting the reports sensibly

When you open a Winter or Summer Outlook, read it in a fixed order. Start with the assumptions: the weather condition used for the demand forecast, the treatment of interconnectors, and the outage assumptions behind the de-rated capacity. Then read the margin figure, in both gigawatts and percentage terms, and then the loss of load expectation, which is the number the security standard is written against. The margin tells you the size of the cushion; the loss of load expectation tells you the assessed risk. Headlines that quote only the margin, and call it "tight" or "healthy" without the loss of load figure, are telling you less than they appear to.

Treat the documents as forecasts with stated limits. The Net Zero Technology Outlook carries the explicit statement that "The Outlook is not a statement of government policy"⁴, and the same caution applies to seasonal outlooks: they describe an expected position, not a commitment. Compare like with like across years: the statutory report's winter 2025/26 figures² are calculated on the Average Cold Spell basis, and a margin quoted on a different weather basis is not comparable. Where two documents quote different figures for the same winter, check which weather condition and which de-rating method each used before concluding they conflict.

A few habits help. Read the sensitivity sections, which show how the margin moves under colder weather or higher outages, because those ranges are more informative than the single central figure. Note what the report says about flexibility and interconnector assumptions, since these are the parts most likely to differ from the central case in real time. And keep the publication rhythm in mind: the outlooks are seasonal documents, refreshed ahead of each winter and each summer, so an outlook quoted months after publication may have been superseded.

Finally, keep the outlooks in proportion. They are the most authoritative public statement of seasonal risk, produced by the body that operates the system, and they deserve to be read carefully. But they are one layer in a system with many layers of protection, and a thin margin in an outlook is a signal for the industry to act, not a forecast of your lights going out. The fuller picture of how Britain's supply reaches your home is in UK energy supply.