Great Britain is braced to pay millions for backup electricity supplies to prevent a shortfall on Wednesday evening as the near-total solar eclipse is expected to wipe out solar power generation.

The government’s electricity grid operator is expected to make multimillion-pound payments to secure electricity at short notice before the moon blocks up to 95% of the sun over parts of the country, the most dramatic solar eclipse visible in Great Britain since 1999.

Gas plants are among the generators that are expected to claim payments of more than four times the market price after the grid operator raised an official alarm that it expects there will not be adequate power generation on Wednesday evening.

The Connah’s Quay gas plant in Flintshire, owned by the German utility company Uniper, agreed to generate electricity on Wednesday afternoon for a price of £875 per megawatt-hour, compared with the market price of just below £212/MWh for this evening.

The payment suggests that the gas plant may be kept running through the evening to help meet the rising demand for electricity, possibly at even higher prices. The darkening of the skies is expected to wipe out between 300 megawatts (MW) and 1,100MW of solar power, just as the fifth heatwave of the summer raises demand for electricity to run air conditioning, fans and refrigeration systems.

The National Energy System Operator (Neso) had initially anticipated an overall shortfall of more than 1,700MW of electricity when compared with the grid’s peak evening demand, according to a warning issued on Tuesday night, or the equivalent capacity of a solar farm the size of 8,500 football pitches.

Under a second market notice issued on Wednesday morning, it said the shortfall remained about 1,200MW.

A spokesperson described the warning notice as a “routine tool” that meant it was asking the energy market to make any additional capacity the generators might have available.

It added that the margin notice was a “precautionary measure to the market” because of the solar eclipse and there was “no risk to customer electricity supplies”.

The grid operator later cancelled the alarm. Market data shows that it requested electricity via the high voltage power cables linking the UK to continental Europe totalling 1,200MW for an hour from 4pm this afternoon, 2,550MW between 5pm and 6pm, and 1,200MW between 7pm and 8pm, according to Timothy Santonastaso, an electricity expert at the market data provider Argus Media.

Electricity margin notices are relatively common in winter when energy demand is higher, but it is the first time that supply warnings have been issued in summer because of the frequent heatwaves, according to Jess Ralston, the head of energy at the Energy and Climate Intelligence Unit, a thinktank.

The soaring temperatures across Europe are being made worse and more common by the climate crisis, she added.

“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,” Ralston said.

Europe’s electricity supply squeeze could become tighter as fresh weather forecasts indicate that above-normal heat could persist across Europe into September, according to forecasters at Amperon.

The analysts warned that already more than 11 gigawatts of European nuclear capacity was currently offline, or operating at reduced output, because of low water levels in rivers used to cool nuclear reactors.

“At the same time, low water levels are complicating coal deliveries, gas plants are approaching the September maintenance season after a demanding summer, and high temperatures are forcing some transmission lines to operate below full capacity,” the forecasters said.

European gas prices have climbed towards their highest levels since the start of the Iran war because of the disruption to Middle East energy exports and rising demand for gas power to meet the demand for electricity to battle the heat.

The power supply squeeze has prompted Britain’s biggest household electricity supplier, Octopus Energy, to encourage customers to avoid running washing machines and dishwashers between 6pm and 8pm on Wednesday to help save electricity.