UK Nuclear Energy Output at Record Low as Gas Fills the Gap

A decade of decline in Britain's nuclear fleet is quietly undermining its clean energy transition, with gas generation rising to compensate and businesses bearing the cost.

The UK generated a record 52.5% of its electricity from renewables in 2025, the second consecutive year above the 50% threshold.

While that figure is impressive, what sits beneath it might concern some in government.

Official figures from the Department for Energy Security and Net Zero (DESNZ) show that nuclear generation fell 12% last year to just 35.9TWh – roughly half the output recorded a decade ago – while gas generation rose 4.7% to 91.6TWh, making it the single largest source of electricity on the grid at a 31.5% share.

Wind, despite its record performance, came in second at 30%.

Total fossil fuel generation rose 2% on the year, driven almost entirely by gas.

A pattern or an anomaly?

The data reveals a paradox that goes to the heart of the UK’s energy transition.

Renewables are genuinely expanding, but they are intrinsically variable by nature – output rises and falls with wind speed and sunlight, not with demand.

By contrast, nuclear is more akin to gas insofar as it can provide consistent baseload power around the clock. Despite the demand, the UK's ageing nuclear energy infrastructure has been steadily losing that capacity.

As older plants have been decommissioned and unplanned outages have mounted across those still operating, gas has filled the void. This is less because the UK is choosing fossil fuels, but because there is currently nothing else that can respond at scale and at such short notice.

John Haw, CEO of energy procurement firm Fidelity Energy, puts it plainly.

“The record renewable figures are genuinely impressive, but they're masking a problem,” he says. 

“Every time a nuclear plant goes offline for maintenance or decommissioning, gas turbines spin up to compensate. 

“We're building a clean energy system on top of a baseload void, and until that void is filled – whether by new nuclear, grid-scale storage, or something else – gas isn't going anywhere."

The final quarter of 2025 illustrated the trend at its most acute.

Nuclear output fell 13% in Q4 to just 8.3 TWh as outages continued, a pattern that analysts expect to persist as the existing fleet approaches the end of its operational life.

The cost for businesses

The consequence of increased gas use is, of course, environmental, but it is also contributing to the kind of price volatility that British consumers have experienced with energy providers in the past few years.

Naturally, these peaks and troughs in price affect businesses too.

“What this means for businesses is that wholesale electricity prices remain far more gas-exposed than the renewable headline figures suggest,” John explains.

“A company that thinks the grid is now 'mostly green' and therefore stable may be in for a rude awakening the next time gas prices spike,” he adds.

Energy procurement strategies built on the assumption of a steadily decarbonising grid – without accounting for the baseload gap – could be underpricing risk.

Britain's nuclear future

Most analysts – and the UK Government – agree that the answer lies in building more nuclear power plants across the country.

Hinkley Point C, the EDF-led plant under construction in Somerset, is not expected to generate electricity until the late 2020s at the earliest. 

What’s more, the project’s costs and delays have had political commentators like Sara Stefanini describing it as a “headache” for around a decade now.

More recently, though, the Labour government has moved to back small modular reactors (SMRs) as a faster, potentially cheaper route to new nuclear capacity, with GB Energy striking deals with Rolls-Royce SMR for a series of installations in North Wales.

The approach is a recognition that the traditional model of large-scale nuclear construction – while still pursued at sites like Hinkley and Sizewell – carries risks that have proven difficult to manage in the British context.

France, which generates around 70% of its electricity from nuclear power, offers a stark comparison. The French fleet was built over decades of consistent state investment that now gives it some of the lowest-carbon and most stable electricity prices in Europe.

Whether SMRs can deliver at the pace and scale the UK needs remains an open question.

In the meantime, the numbers from 2025 are a reminder that the transition, for all its genuine progress, still has a long way to go.