In a control room in Wokingham, Berkshire, staff work around the clock, minute by minute, to adapt Britain's fluctuating electricity supply to ever-changing demand.
But as households struggle to adapt to rising summer temperatures, workers at the state-owned National Energy System Operator (Neso), whose job it is to keep the lights on, have in recent weeks faced the kind of challenge usually reserved for the country's coldest winter nights.
The high-pressure heat dome that hit Europe in late June caused wind turbines to slow down, gas-fired power plants to stall and French nuclear plants that exported electricity to Britain to shut down. Meanwhile, electricity demand for air conditioning and refrigerators soared as Brits tried to stay cool.
The task of keeping the grid's power supply stable during the heatwaves in June and July came against a backdrop of a political row and culture war over Britain's ambitious clean energy targets and the costs of reaching net zero.
Earlier this month, Shadow Energy Minister Claire Coutinho told Parliament that Neso whistleblowers had accused senior managers of being involved in a cover-up over the extent of stress on the electricity grid during record temperatures in the UK and mainland Europe at the end of June.
The allegations also included allegations that control room personnel were ordered by senior managers to forego normal note-taking to avoid producing a full audit trail of their efforts to stabilize the energy system. Whistleblowers have also alleged that members of the corporate affairs team tried to pressure operators to make decisions that would protect the organization's reputation, Coutinho said.
Employees came forward because they were “concerned that the power grid was becoming increasingly unmanageable” and that senior bosses were “risking power outages to protect Neso's reputation,” she said. “If that’s true, it’s nothing short of a scandal.”
Last week, Neso said it had brought in independent investigators from law firm Eversheds Sutherland to look into the allegations.
So what do we know about how the grid operator handled the heat wave? The whistleblower allegations focus on the last week of June. It was a week that showed that summer heatwaves “can now create system conditions traditionally associated with winter scarcity events,” according to consultancy LCP Delta.
This week Neso issued a series of emergency measures to safeguard the UK's electricity supply, marking the first use of these instruments during a summer period. Underlining the challenge, this week's figures showed that Neso spent almost £800,000 between mid-April and mid-July trying to get consumers to use less electricity than normal, according to the Financial Times.
Neso took emergency measures to secure the UK's electricity supply in the last week of June. Photo: Tommaso Altamura/Alamy
Despite these levers, the network's frequency – which Neso wants to keep at around 50 Hz – fell to the edge of its operating limits between 49.8 Hz and 50.2 Hz.
The operational area, which is used to prevent power outages, was under pressure for almost an hour on June 23rd. The lowest recorded frequency was 49.66 Hz, still well above the legal lower limit of 49.5 Hz.
However, a former employee told the Guardian that the longer period of lower frequencies than usual could indicate that the network operator was running out of politically acceptable options to deal with the strain on the network before more serious interventions, such as shutting down factories, might have been necessary.
“When I saw the grid frequency data, I thought that it simply couldn't be right because it had been at the lower end of the operating limits for so long. A 0.2 Hz change may not seem very large, but technically it represents a much greater vulnerability. At these values there is a risk that something unexpected could happen that would place too much stress on the system to absorb,” they said.
Neso downplayed the concerns. Although the heatwave resulted in “ongoing pressure on power systems” and “tight operating margins”, according to Craig Dyke, a director at Neso, “no customer demand was interrupted, frequency remained within legal limits, voltage remained within limits and no lines or cables were overloaded”.
In two cases in the last week of June, the grid operator was forced to sound the alarm on power supplies due to the effects of the heat wave, which scientists said was more severe and more likely to occur due to the climate crisis.
This resulted in payments estimated at millions of pounds for just a few hours of emergency power from gas-fired power stations, a major cause of atmospheric carbon emissions that are accelerating global warming.
Neso controllers have also exceeded normal limits for electricity trading over the vast undersea power cables that connect Britain's power system to generators in continental Europe “several times” a week, analysts said.
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Neso controllers are understood to have exceeded normal limits for trading electricity over the subsea power cables that connect the UK power system to generators in continental Europe. Photo: Mint Images/Getty Images/Mint Images RF
The largest of these breaches occurred on June 24, when Neso issued an emergency call for an additional 500 MW of electricity capacity from the Netherlands via the BritNed power line, which was achieved by cutting UK exports. A similar distress call went to France.
Some market experts are concerned about the idea that there may have been a “near miss” power outage in the UK during the heatwave. Power outages are extremely rare in the UK and when they do occur, they are the result of a combination of unforeseen events.
Europe's worst blackout in 2025 – in Spain, Portugal and parts of southern France – was blamed on several converging factors that destabilized grid frequency. In 2019, the UK experienced a short-lived blackout when a lightning strike caused two power generators located more than 100 miles apart to be disconnected from the grid in an “extremely rare and unexpected event.”
The former employee emphasized that the risk to Neso's reputation from a possible near miss was significant. In a worst-case scenario, Neso can resort to lowering grid voltage to prevent a widespread power outage, or even shutting down large industrial energy consumers to keep the lights on in homes across the country. “But that's what I hope you never have to do. That's the last thing you want,” they said.
In 2019, the UK experienced a short-lived power outage when a lightning strike caused two power generators to be disconnected from the grid. Photo: Yui Mok/PA
The pressure to avoid reputational damage would be nothing new to those in the Wokingham control room. Neso became a state-controlled entity after the government bought it from FTSE 100 energy company National Grid for £630 million in late 2024.
Under the previous ownership, engineers were aware that a misstep could hurt National Grid's stock price. Now they face the burden of expecting to smoothly balance supply and demand as more clean energy sources come online and Labor pushes to meet a green energy target by 2030, the former control room official said.
“Whether or not there is a direct order to cover up a scandal, individuals may be placed in an awkward position that could prevent them from taking the right action. There was always a sense of being cautious about how your actions might be viewed by superiors. That could mean failing to take timely action.” [to rebalance the system] if you are concerned that this could cause senior managers to look askance,” they added.
In a letter to the UK's industry regulator Ofgem, Fintan Slye, Neso's chief executive, said the network operator had “policies and procedures in place” to ensure this type of influence cannot take place. “Nevertheless, we take these allegations extremely seriously,” he said. A spokesman said the Eversheds investigation would be shared with Ofgem and the government and eventually published. It is unclear when exactly that will be.
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