"As climate change has more of an effect we could see demand patterns change as we have hotter summers and warmer winters," a spokesman for the UK arm of Germany utility E.ON said.

"That's something that we're already looking at because it could well inform future investment decisions and the way we run our business in future decades."

The vulnerability of nuclear power stations to extreme heat became all too clear last month as reactors in France and Germany had to rein-in production because the river water they need to cool down was too warm.

In Spain, more used to the heat than northern Europe, most reactors kept running through July, but the oldest reactor was still forced to stop because of tepid river water.

Europe's gas and coal-fired plants also laboured in the heat and lower river levels.

All these technologies may have to be altered as hotter, drier weather in future looks increasingly probable.

"Output from many forms of generation, especially gas-fired, is reduced in hot weather," according to a UK Met Office report on the impact of climate change.

"In a future hot dry summer, if the volume of water in a river that supplies cooling water was markedly reduced, it is possible that a power station would have to reduce output."

At the same time that hot weather impacts power supply it also pushes up demand as air conditioning and refrigeration units go into overdrive.

HYDRO POWER

Hydropower is also at the mercy of the effects of climate change.

Unusually dry weather in Scandinavia this week helped fire power prices to new highs as traders eyed well below average water levels in reservoirs feeding hydro-electric plants, which provide half the Nordic region's electricity.

Norwegian reservoirs stand at just over 60 percent full -- about a fifth lower than typical levels for the time of year. Sweden's reservoirs are about 19 percentage points below long-term median levels for the time of year.

Spain's experience of severe drought last year shows how much power generation can be influenced by the weather.

Spain typically gets about 12 percent of its electricity from hydro. But during one of its driest years on record, last year Spanish hydro production fall to 55 percent below the long term average.

This forced Spanish generators to increase coal, gas and fuel-oil fired power generation, which pushed up power prices and sent generators scurrying off in search of more carbon emissions rights.

Although Spanish hydropower reserves this week are up 14. 6 percent on where they were this time last year, they are still more than 20 percent below the average over the last 10 years.

However, Spain's hydro plants are designed to endure a few dry years, according to Oscar Arnedillo, director of NERA Economic Consulting in Madrid.

"In Spain we have some hydro plants which are capable of regulating their flows across years. So we have some capability to smooth output across cycles," he said.

Switzerland's hydro plants have also ridden out this year's heatwave.

"The summer heat also helped melt glaciers very quickly which boosted Alpine water reservoirs levels and thus production at stored-water plants up in the mountains," said an official at the power industry association VSE.