The spike in electricity prices this week because of the hot weather was a stark illustration of how global warming and the growing demand for air-conditioning are changing the UK's energy market.
Temperatures hit a July record in the south of England yesterday, reaching 36.3°C at Gatwick airport, and retailers reported a rush on fans and air-conditioning units.
The summer has traditionally been a time of low demand for electricity, when generators can take power stations out of service for maintenance. But on Tuesday, the spot price of electricity tripled as the National Grid, the energy distributor, warned that supply was getting tight.
Electricity prices retreated again yesterday after additional power stations were fired up to boost supply. But the episode has been a salutary reminder to energy companies of the need to make sure more electricity is available in the summer months.
"We are a long way off the situation in places like the US and southern Europe, where the peak of energy demand occurs in the summer," said National Grid. "But more energy is being used in the summer; we British are getting more used to using air conditioning."
At temperatures above about 19°C, the demand for electricity rises. At midday on Tuesday, when the national average temperature was 24°C, demand was 2,000 megawatts higher than the same point a year ago, when the national average temperature was just 18°C.
Blackouts are not yet an imminent threat: there are several measures that can be taken to increase supply and curb demand.
National Grid can ask large power users to limit their consumption. Power distributors can curb their usage by turning down the voltage supplied to households and businesses. But the first way to bring supply and demand into a better balance is the response to market signals: high prices bring more generators on stream.
The surge in demand caused by the hot weather has brought even higher-cost generators back into operation. For instance, the UK's three oil-fired power stations, owned by Eon UK and RWE npower, have all been fired up this week, which is highly unusual for the summer months. These plants are expensive to run and are generally turned on only in the winter when energy demand is at its peak and high electricity prices make their operation worthwhile.
Eon UK, owner of Powergen, runs the oil-fired plant ot the Isle of Grain, Kent, while RWE npower owns the oil-fired plants at Fawley in Hampshire and Littlebrook, Kent.
Some electricity contracts rose as high as £260 per megawatt hour on Tuesday evening, when supplies were at their tightest. Such high prices would have to be sustained for some time before power suppliers put up bills for residential customers. Businesses are more at risk of being immediately affected, although they are often protected by fixed-price contracts.
Martin Brough, head of energy at Oxera, the consultants, said power-generating companies would have to get used to spreading their maintenance work through the year, rather than doing it all in the summer.
That could lead to more shortages at other times of year if there are unplanned power station shutdowns.
The implication is that the country could need more power generation capacity to get through the peaks in demand safely.
It seems as though this is going to be a permanent change. By 2040, the Met Office believes, summer temperatures such as those recorded yesterday could be the rule, not the exception.
