France has urged firms to use less electricity as a heatwave across Europe triggers high demand for power supplies.
French firm EDF had to purchase 2,000 megawatt hours of electricity from abroad to make up for its shortfall.
Meanwhile Britain turned to expensive oil-fired stations to meet demand but said enough supplies were available.
Wholesale energy prices on Wednesday were on a par with Tuesday's high in France, the UK and Germany.
Investment needed
Increased use of air conditioners and refrigerators, coupled with lower power output at hydro-electric and nuclear power stations in France, contributed to the squeeze for demand.
The 2,000 megawatt-hours France was forced to import was the equivalent capacity of two large nuclear stations. Analysts speculated that the power had come from Germany.
The heatwave underlined the need for EDF to invest to ensure energy security said Pierre Gadonneix, EDF chairman.
The firm plans to invest 20bn euros (£13.6bn) in distribution, transport and production.
EDF said it was concentrating on coastal plants, which cope better with rising temperatures than inland plants.
'Continued volatility'
Meanwhile in the UK, the heatwave has coincided with planned maintenance, compounding the pressure on supplies and forcing the price of spot electricity prices up three-fold on Tuesday.
Britain's National Grid urged suppliers to restart some power stations which had been closed for repair work.
Npower has started up oil-fuelled power stations at Fawley, Southampton and Littlebrook in Kent. Centrica has upped output from its station at South Humber in Lincolnshire.
Britain is sweltering on the hottest day of the year so far.
Meteorologists recorded temperatures of 36.3C (about 96.8F) south of London, making it the hottest day since 1911.
On Tuesday the National Grid issued a statement saying that while there was enough electricity to meet demand, it was not happy with the surplus availability.
This warning was cancelled at 6am on Wednesday after the problem was overcome by extra supply becoming available.
British Gas owner Centrica said it was impossible to predict whether the heatwave would produce a long-term effect on electricity bills.
"This demonstrates the continued volatility in wholesale energy markets which all suppliers are faced with at the moment," said spokesman Andrew Hanson.
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