Great Britain will need gas-fired power plants well into the next decade to shore up supplies, with SSE Plc now expecting most of its fleet to remain open into the mid-2030s despite the rapid expansion of renewable energy.

The outlook marks a shift for the Scottish utility, whose Chief Executive Officer Martin Pibworth had previously expected three of its large gas plants to reach the end of their operating life by the end of this decade as the nation moves toward a cleaner power system. Many of the nation’s gas plants were built in the late 1990s and operators are facing rising maintenance costs to keep them in use.

The change underscores the challenge of keeping the lights on as Britain becomes increasingly reliant on intermittent wind and solar. While batteries and demand-side response measures are also expanding quickly, they can’t compete with the speed gas-fired plants can fire up.

“I would expect most of our gas plants would still be functioning in the mid 2030s as the reality of needing that flexibility, but functioning on much lower load factors going forward,” Pibworth said in an interview at Bloomberg’s London office.

Pibworth’s comments come as uncertainty hangs over the government’s target for at least 95% of electricity generation to come from clean sources by 2030. Energy Secretary Miatta Fahnbulleh has reaffirmed her commitment to a “sprint to clean power.”But she has described the goal - set under her predecessor Ed Miliband- as a stretch and stopped short of backing the 95% figure.

Reducing reliance on unabated gas could become harder if the government scales back support for carbon capture. Ministers are considering cuts to CCS as the energy department looks for savings, Bloomberg reported Friday.

Yet, with just a few years to go until the end of the decade, gas-fired plants are still contributing by almost a quarter of the power mix.

As the stations run less frequently, but remain essential as backup, payments to keep them available will become much more important for their economics, according to SSE. The nation operates a capacity market that pays generators to ensure their plants are always available when the system needs them.

But the reliance on gas-fired power will come at a cost. UK day-ahead power prices averaged about £153 per megawatt-hour during periods when gas supplied at least 40% of generation in the first nine months of the year. That’s about 41% above the overall average, according to Bloomberg calculations using grid data.

Pibworth, who got his start as an energy trader at SSE almost 30 years ago, pointed out that spark-spreads, the measure of profitability by generating power from gas, are negative for this winter as well as next summer. That would indicate a lower incentive for operators to invest in new gas plants, he said.

It also highlights the challenge for Andy Burnham’s government which has put energy affordability at the heart of its agenda. Last month, Fahnbulleh told the Bloomberg Green Summit in New York that affordability is her top priority and above climate change. But she also argued that they require the same solution — reducing the nation’s reliance on gas-fired power plants.