Wind farm operators were paid £2.6m to keep their turbines idle last month in the latest stealth charge on household power bills.

'Scottish Power, which last week announced price rises averaging 10% for electricity and 19% for gas, was one of the firms paid to switch off turbines. It received approximately £720,000.

'National Grid, which operates Britain's main power and gas transmission networks, admitted it made the payments, which are paid for by the consumer.

'It blamed lulls in demand during May, an unusually warm month, which meant there were times when energy generated by wind farms was not needed. It said efforts to update the power network had not kept pace with the construction of wind farms.

'When The Sunday Times first revealed last month that wind farms were being paid to turn off their turbines, the National Grid claimed the £900,000
payments were exceptional. Charles Hendry, the energy minister, also described them as an "anomaly".

'Now research carried out by the Renewable Energy Foundation (REF) reveals four energy companies were paid to switch off turbines on 16 occasions in May.

'The most expensive shutdown occurred on May 24, when seven wind farms came off-line for a total of 69 hours at a cost to the grid of £613,000.

'The 52-turbine Hadyard Hill wind farm in south Ayrshire was affected more frequently than any other. Its operator, Scottish and Southern Energy, was
paid a total of £1m to shut the site for the equivalent of six days in May.

'Scottish Power was paid about £720,000 for shutting three wind farms, including Whitelee in Ayrshire, the largest onshore wind farm in Europe. RWE
npower renewables was paid about £573,000 and Falck Renewable more than £246,000.

'On one occasion infrastructure problems meant Scotland needed to buy energy from England while - perversely - two Scottish wind farms were paid about £34,000 to power down.

'The payments raise questions about whether thousands of new onshore turbines planned for Scotland can be justified.

'National Grid defended the £2.6m payments, saying wind farm operators were paid to shut down as their output could be more easily managed than other energy providers, such as nuclear plants.

'John Constable, of the REF, said: "It is clear the grid is struggling to integrate the existing wind farms at reasonable cost to the consumer. There
appears to be an expensive mismatch between political rhetoric and engineering reality."