Introduction
On this Substack we have often criticised NESO. The criticisms have focused on the strategy and policy output, for example the six impossible things we were expected to believe before breakfast about the Clean Power 2030 plan and the phantasmagorical Future Energy Scenarios.
However, it now appears the wishful thinking infecting NESO’s policy-making has spread to the managers of the vital operations that keep the lights on every second of every hour of every day. Shadow Energy Secretary, Claire Coutinho, has been approached by several whistleblowers, presumably workers in the control room, worried that the electricity system is not being run securely. The credibility of NESO has certainly taken a knock as a result of these allegations. Time for a deep dive into NESO’s credibility outage.
NESO Objectives
NESO is the supposedly independent systems operator (ISOP) for both the electricity and gas systems in the UK. The Energy Act 2023 defines three objectives for the ISOP as:
The net zero objective
The security of supply objective
The efficiency and economy objective
Although the statute does not give particular priority to any of the objectives, it is illuminating to see the Net Zero objective is placed first on the list. Back in 2019, when still just ESO, CEO Fintan Slye announced they would “be able to fully operate Great Britain’s electricity system with zero carbon by 2025.” Although they have not achieved that goal, earlier this year they gleefully announced they had run the grid at 98.8% zero carbon for a short period. It certainly appears as though the Net Zero objective is foremost in their mind.
The security of supply objective should be paramount because if the lights go out, people will die. If it is cold, heating systems will fail and the vulnerable will die of cold. If it is hot, air-conditioning systems will fail, leaving the vulnerable potentially exposed to dangerous heat. It could be catastrophic if Net Zero goals are placed above security of supply. If experienced grid operations professionals are concerned, then they deserve to have their concerns taken seriously.
Whistleblower Allegations and Response
The allegations made to Claire Coutinho are threefold. First, on 23 June, NESO failed to meet standards on constraints and reserves put in place to prevent blackouts. Second, the Corporate Affairs team interfered with operational decisions, prioritising NESO’s reputation over security of supply. And third, operational decisions are being kept in ‘live documents’ with no audit trail, therefore preventing them from being accessed for Freedom of Information requests.
It is instructive that concerned grid engineers felt the need to express their concerns to the Shadow Energy Secretary and not go through internal channels. This lack of confidence in NESO management is the first strike against NESO credibility.
NESO has responded with a long thread on X and an explanatory note on its website. The thread (post 9 of 11) acknowledged “input” from Corporate Affairs; some might consider this input to constitute interfering. The thread also asserted (post 4) that “no customer demand was disconnected” and “frequency remained within statutory limits.” They also said they are conducting a full analysis of market and system operation during extreme weather. It appears as though this will be an internal investigation where NESO marks its own homework, although Ofgem now seems to have taken charge of this investigation. Separately, an independent investigation, run by Eversheds Sutherland, will look into the poor record-keeping claims and allegations of improper influence on decision making. Crucially, the independent investigation will not look into whether security standards were breached. This is the second strike against NESO’s credibility – surely the independent investigation should extend beyond checking that the appropriate boxes were ticked at the right time into the fundamentals of grid security. Process should not come above outcomes.
It has also been alleged that in an all-staff meeting Fintan Slye dismissed the allegations as false and that other NESO senior managers told staff the whistleblowers had “let down” the company. Both allegations confirmed by recordings published in the Telegraph. Now the The Times (full article here)claims to have seen internal reports contradicting Slye, saying there were five separate constraint breaches across the network, system security was compromised and they didn’t have even 1MW left to instruct. This is the third strike against NESO’s credibility – senior management should not lie and whistleblowers should not be denigrated; their concerns should be listened to and addressed.
Extended Frequency Drop on 23 June
On 23 June there was an extended period where the grid frequency dipped below the normal operational limit of 49.8Hz, as shown in Figure 1, from NESO’s System Frequency Data for June 2026.
NESO provides data for each second. For the chart, the data has been summarised to show the maximum frequency for each minute. As can be seen it is not uncommon for there to be short excursions close to or below the normal operating limit of 49.8Hz. However, as shown in the red oval, from 17:53 to 17:59 and then again from 18:05 to 18:28 the maximum frequency never went above the minimum threshold. There was another dip in frequency from 18:50 to 19:03 when the minimum frequency reached as low as 49.657Hz at 19:00. When this happens the grid operators try to dispatch extra generation (or reduce load) to increase the frequency.
The extended period where the frequency was below the normal operating limit indicates something was not right with the grid. According to the report of the Times article linked above, there were “five separate constraint breaches across the transmission network, each exceeding the limit by hundreds of megawatts, with the largest breach reaching 700MW.” Apparently, there was no capacity left to instruct, so grid operators were left with little room for manoeuvre. NESO were strictly correct in saying the statutory limits on frequency were not breached, but they glossed over the constraint breaches and the extended period where frequency was below the normal operating limit.
Interestingly, no Electricity Margin Notices (EMNs) were issued ahead of this event which indicates NESO did not foresee the problem ahead of time. It appears as though emergency assistance was invoked by cutting exports to the Netherlands and France to reduce load and increase frequency. This is the fourth strike against NESO’s credibility because they claimed safety margins were not breached and no customer demand was disconnected. It looks like the Netherlands and France do not count as customers in NESO’s eyes.
Unprecedented Electricity Margin Notices
NESO is duty bound to maintain a safety margin of generation capacity to cover situations where a generator or transmission link fails. When NESO foresees margins may become tight it issues EMNs to encourage extra capacity to be made available. Since the events of 23 June, NESO has issued EMNs for 24 June, 26 June and 9 July. As discussed below, almost all the data for 24 and 26 June is missing from NESO’s published inertia data. EMNs are relatively rare, but it is unprecedented for such notices to be issued in summer when demand is generally much lower than in winter.
These additional EMNs indicate that there are hidden problems under the covers. There are potentially several factors at play. First, low wind conditions reduce wind generation; second conventional and solar generating efficiency is reduced in hot weather; third there are possibly more generators offline due to maintenance; fourth battery backup may not last long enough to cover extended problems and finally transmission capacity in some areas may be curtailed to avoid overheating in the hot weather.
The unprecedented timing of these EMNs is the fifth strike against NESO’s credibility because they have not prepared properly for foreseeable hot summer weather conditions.
Inertia Drops Below Lower Limit
In the past, inertia from large spinning turbines in gas and coal power stations prevented rapid changes in frequency on the grid. Now with much more wind and solar on the grid, the natural inertia on the grid has reduced significantly. Ofgem and NESO set the minimum inertia levels and this has reduced from 140GVAs in early 2024 to 120GVAs today with plans approved by Ofgem to reduce the minimum first to 110GVAs and then to 102GVAs later this year or early 2027. Less inertia gives grid operators less time to react if rapid changes in frequency occur.
NESO provides half-hourly system inertia data that shows the total inertia outturn and the amount of inertia provided by the market. The data since 2021/22 is shown in Figure 2.
The blue dots are the outturn inertia for each half hour period for each day. The pink area shows the inertia lower limit in force for each day. The outturn data show that, with seasonal variations, the maximum inertia on the grid is falling. Inertia sometimes falls below the minimum level, however the small transgressions may be explained by the inertia outturn not including the contribution from stability pathfinder units. These pathfinder units are typically large rotating flywheels and condensers to provide inertia without generating electricity or batteries providing synthetic inertia. Separate data from NESO shows the Pathfinder units typically ~10GVAs. The very large falls below the minimum on five of the days days may possibly be explained by data glitches. In addition, it also looks like maximum inertia in Spring/Summer 2026 is running below that of other years.
Zooming in on the data for just 2026 shows a deteriorating picture in June, see Figure 3.
On 1 June, minimum inertia was recorded as just 43GVAs, but we may be able to dismiss that as a data glitch. The red circle shows that since 17 June, there have been many excursions below both the current lower limit of 120GVAs and the future lower limit of 102GVAs. The scale of the breaches is much greater than can be explained by the absence of the ~10GVAs contribution from the pathfinder units. The minimum inertia was 81GVAs on 17 June, 86GVAs on 19 June, 89GVAs on 21 June and 91GVAs on 27 June. On almost half the days in June, 46 of the expected 48 daily records are missing, with just 2 entries recorded. It is therefore possible the lower limit was breached on other days too. The missing data days begin to appear in mid-May.
Interestingly, NESO has paused giving updates to the inertia dataset, with no data available beyond the second settlement period of 30 June 2026 (see Figure 4).
NESO says they “actively embrace the need to share our data, fostering transparency, innovation, and collaboration.” They go on to say their data sharing approach “prioritises sharing as much data as possible through our Open Data Portal.” Multiple breaches of the low inertia limit, missing data and pausing updates to the inertia data is the sixth strike against NESO’s credibility.
The lack of data is not the only problem with inertia. The gap between market inertia and outturn inertia is closed by actions taken by NESO to manage the grid. The problem is that the proportion of inertia provided by the market is falling compared to total outturn inertia (see Figure 5).
The proportion of inertia provided by the market has fallen from around 95% in early 2021 to below 77% in late June 2026 and the trend is clearly down. This means that the gap between the market and the outturn is growing as shown in Figure 6.
The gap has doubled from roughly 10GVAs in 2021 and early 2022 to around 20GVAs in mid-2026, with average weekly highs sometimes over 30GVAs. The low inertia outturns in late-June (Figure 3) perhaps indicate that NESO was struggling to take effective action to bridge the gap between market inertia and the required minimum. The task of the control room is of course made more difficult because they now have to interact with hundreds, potentially thousands, of generators and service providers, whereas in earlier days they had at most a few dozen power stations to deal with.
This matters because part of NESO’s justification for reducing the minimum inertia limit to 110GVAs and then to 102GVAs relies on procuring more dynamic response services as more wind and solar come on to the grid and fewer conventional power stations are dispatched. The approval for this change has had a chequered history. Ofgem refused NESO’s first attempt on the grounds that NESO had not provided sufficient detail to underpin their recommendations. Ofgem also noted that although the Security and Quality of Supply Standard (SQSS) committee had voted to support NESO’s recommendations, three of the seven panel members had abstained from voting, indicating dissent in the ranks.
This is the seventh and final strike for NESO’s credibility. Their plans to reduce inertia on the grid rely on procuring more synthetic inertia to close the gap from market inertia, but it appears as though in June they struggled to close the gap and failed to meet the minimum inertia limit on many occasions.
Ministerial Credibility Outage
This furore has reached Parliament too with Claire Coutinho granted an urgent question. Unfortunately, as the video clip below shows, the Minister Michael Shanks’ answer amounted to little more than “everything is fine, nothing to see here” (from 6:10) because he appeared to deny that operational limits have been breached. He may wish to correct the record or he might face charges of misleading the House.
Conclusions
Stability of the grid and the provision of electricity-on-demand is a fundamental underpinning of modern society. Widespread interruptions to supply or even worse a countrywide blackout like that seen in Spain last year could be catastrophic.
It is clear that experienced grid operators are so concerned about what is going on they have felt the need to go outside NESO and blow the whistle to Claire Coutinho. This is the first strike against NESO’s credibility. NESO’s attempt to denigrate the whistle blowers is another strike. It is outrageous that NESO management should claim the whistleblowers have somehow let the company down when they are clearly trying to save us all from potential catastrophe. The scope of the independent investigation also leaves a lot to be desired.
The low frequency events on 23 June and the multiple breaches of the inertia lower limits in late June indicate that the grid operators are right to be concerned. These incidents constitute further strikes against NESO’s credibility and the “pause” in providing inertia data to obstruct independent analysis is another indictment against them. Now we learn there were sufficient concerns in the SQSS committee for three of the seven members to withhold their vote to approve a further reduction in grid inertia. Although now approved, NESO’s plan appears to rely on procuring synthetic inertia that failed to deliver enough in late June to close the gap between market inertia and the required minimum. After seven strikes, it must surely be time for some of the NESO senior management to be out.
Wishful thinking and Net Zero ideology will not keep the lights on. Security of supply must rank higher than Net Zero. Always. It is to be hoped NESO’s credibility outage is restored before the country endures a significant power outage.
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Sadly my article from October February 2024, is turning out to be somewhat prescient.
https://davidturver.substack.com/p/wait-for-the-blackout?r=nhgn1&utm_campaign=post-expanded-share&utm_medium=web
NESO’s enthusiasm for Net Zero is, naturally, commendable, it does appear that their recent initiatives — including the imaginative decision to avoid permanent records, the adventurous lowering of inertia limits, and the spirited attempt to rely on synthetic inertia that unfortunately failed to materialise when required — may collectively constitute what one might delicately describe as a system‑wide flirtation with catastrophe.
Given that the grid operators themselves have felt compelled to seek external assistance, and that the SQSS committee has expressed reservations of a magnitude rarely seen outside discussions of experimental aircraft, it may be prudent to consider whether NESO’s current strategy is less a plan for energy security and more a blueprint for a countrywide blackout.
I merely ask, whether Ed Miliband is entirely confident that NESO’s present course — guided as it is by ideological enthusiasm rather than the tedious constraints of physics — will keep the lights on, rather than provide the nation with an unplanned demonstration of what life was like before electricity.
As it currently stands, the Government had better issue each and every household a guidance note of what to do as and when the National Grid shuts down- Prepare to Survive, with handy tips to have candles, tinned food, batteries, a windup radio, a windup phone recharger, with the helpful reminder that the Nation could be back to full power within 4 to 16weeks………….meanwhile Local Government will offer guidance but no assistance (a re run of the Nuclear attack and survive).