Executive Summary

• The National Energy System Operator (NESO) issued its fourth electricity margin notice (EMN) of the summer for the recent solar eclipse
• While it is a normal operational tool – and while a solar eclipse on this scale is an extremely rare event – the EMNs amid the summer heatwaves showcase the balance required with cleaner energy
• Jess Ralston, of the Energy and Climate Intelligence Unit, said that ‘the only way to bring our climate back into balance is to reach net zero emissions, including rolling out more renewables and upgrading the grid after decades of under-investment’

The recent solar eclipse made for something of an interesting glimpse of the UK’s renewables mix – and the opportunities and problems it faces.

The timing of the eclipse on 12 August, set for between 6pm and 8pm for the vast majority of the UK, was particularly crucial here. It meant a chunk of Britain’s solar power – up to 1.1GW, according to one account – would be knocked out just as millions of people were getting home and firing up their appliances. Octopus Energy ran an Eclipse Power Down session for those two hours, and offered a potential carrot for customers who took part, in the form of free electricity the following weekend, as well as a draw for free solar panels and installation.

This was not to be underestimated. The National Energy System Operator (NESO) issued an electricity margin notice (EMN) for the eclipse – its fourth EMN of the summer. While it is a normal operational tool used to notify the market that additional generation capacity may be required, since NESO began operating in October 2024, it had only issued two EMNs, both in January last year. January and February are naturally the most at-risk months for Britain’s electricity supplies. “No panic yet, but three emergency notices on the grid is a bad sign,” The Times wrote in July.

Four is not a record for a season, however. In the days when the National Grid Electricity System Operator was in charge, the winter of 2020/21 saw six EMNs issued between November and February. Each one was cancelled ahead of real-time, and the National Grid noted the extenuating circumstances of transitions in and out of national and regional Covid-19 lockdowns.

The kicker during that winter, however, was ‘particularly extreme’ wind patterns, with periods of very high activity followed by unusual low spells. This, ultimately, is from where the summer EMNs derive. The relentless sunshine and dry weather is of benefit to the solar farms, of course, but the combination of unusually high temperatures and low wind can put strain on capacity, as NESO explained in June. Again, no panic: just an explanation that the operator’s role is to ‘anticipate changing conditions, use the tools available… and maintain a secure and reliable supply of electricity for homes and businesses.’ The 23 June EMN was ‘one example of that in action.’

However, as The Guardian reported last week, Britain was ‘braced’ to pay millions for backup electricity supplies. The Connah’s Quay gas plant owned by Uniper, a German company, agreed to generate electricity on the Wednesday afternoon, hours before the eclipse, for a price of £875 per MWh – more than four times the market price.

While the eclipse can certainly be put under the rare occurrence category, record-breaking summer temperatures cannot, as Jess Ralston, head of energy at the Energy and Climate Intelligence Unit thinktank noted.

“This year has seen more margin notices issued in the summer because of the frequent heatwaves, which are being made worse, and more common by climate change, as extreme heat drives up demand from air con and fans while reducing efficiency of many power sources including gas and nuclear,” said Ralston.

“The only way to bring our climate back into balance is to reach net zero emissions, including rolling out more renewables and upgrading the grid after decades of under-investment,” she added.

In terms of investment, NESO’s most recent Beyond 2030 report, published in June, noted that the overall need for investment in the transmission network, set out in the 2024 paper, remains ‘broadly unchanged’, but that without reinforcement, constraint costs from balancing the system could increase by three times between 2031 and 2035. NESO recommended that, across Britain, a total of £89 billion of network investment was needed ‘delivering a coordinated offshore and onshore network design, supported by targeted upgrades to local networks and power flow control.’

As Octopus noted, solar now represents an increasingly important role in Britain’s energy system, generating a record 14.4% of Britain’s electricity in July. Yet as the several summer EMNs from the national operator indicate, getting the balance right isn’t always easy.

Image credit: ‘Partial Eclipse Cardiff August 2026-8’, by Andrew Rees, used under CC BY-ND 4.0 license