The news comes as STV reports the Green party to have launched its national energy policy, to end “risky” nuclear and “dirty” coal power in favour of "renewables," and to see the country "export" surplus power to other parts of the UK, while Scottish Water and local authorities are empowered to develop renewable capacity in the public sector and reduce dependence on central funding.

BBC News however reports that Renewable Energy Foundation (REF) research finds energy companies were made 'constraint' payments of £900,000 to halt turbines for several hours between April 5-6.

Scottish Power's Whitelee windfarm in East Renfrewshire was paid £308,000, its Blacklaw windfarm in Lanarkshire £130,000 and Beinn Tharsuin £180,000.

RWE nPower's Farr windfarm, south from Inverness was paid £265,000.  SSE Renewables at Hadyard Hill took £140,000 in constraints. And Falck Renewables  Millennium windfarm in the Highlands, received £33,000.

"It seems that Hadyard Hill is the most likely to be asked to curtail power, having reduced output on 5 dates in April 2011. Whitelee has received the largest total sum as a result of being paid to reduce output on 30 May 2010 (the test run), on the 29th of October 2010, and on the 6th of April 2011.

" We estimate the total paid to Whitelee’s owners for these curtailments is approximately £460,000. These payments are in addition to the subsidy payments enjoyed by Whitelee of approximately £28m yearly, and lead to the observation that constraint payments are already likely to be a significant proportion of the total income of some wind farms to date having reached approximately £1m."

Dr Lee Moroney, (right) planning director for the REF, which has criticised subsidies to the renewable sector in the past, said: "The variability of wind power poses grid management problems for which there are no cheap solutions."

The April event occurred because the Scottish grid network could not absorb all the energy generated, and chose to constrain wind power off the system, paying very high prices to compensate wind generators for the lost income, in some cases as high as 20 times the value of the electricity which would otherwise have been generated.

In total approximately £890,000 pounds was paid over a few hours to six wind farms, these costs being ultimately destined to pass on to the consumer.

REF has consistently argued that the scale and pace of wind power development exceeds the ability of the system to integrate this "uncontrollable" energy source, and high costs to the consumer would consequently result.

Writing in the preface to Paul-Frederik Bach’s 2010 study for REF, Professor Michael Laughton observes: "The outstanding major concern in the work reported here, and one with very serious implications – especially for the United Kingdom with its predominantly island system with inadequate international interconnection capacity – is the extent to which subsidised wind power can, in practice, be used within the system, without needing to be constrained off: in other words wasted, or exported at whatever market prices, perhaps disadvantageous ones, prevail elsewhere."

The payments on the 5-6 April confirm these concerns, even at relatively low levels of wind power currently installed in the UK (just over 5GW of capacity) and are a worrying sign of things to come.

REF concludes that the scale and pace of wind energy development in the United Kingdom needs rethinking, with more emphasis placed on provision of economic solutions to the grid-balancing problem.

Some will judge that constraint payments show the grid network, particularly that interconnecting England and Scotland, needs to be expanded. It should be noted that such network enhancement is not cost free, and would have a very significant impact on consumer bills.

Indeed, all the currently available solutions for problems posed by uncontrollable generation such as wind power are expensive.

It is conceivable that invention and innovation could reduce these costs, but at present renewable subsidies and the socialisation of integration costs, mean there is no commercial incentive for technologists to seek less expensive solutions.

REF notes that it is at least arguable that more flexible renewable generators, including dedicated biomass, biomass co-firing, anaerobic digestion, and energy from waste are under-represented in the renewables mix.  This should be remedied, not least because they can be integrated into the system at lower cost to the consumer.

The National Grid said the network had overloaded because high winds and heavy rain in Scotland overnight on 5-6 April produced more wind energy than it could use.

Spokesman, Stewart Larque noted a transmission fault in the system meant the surplus energy could not be transferred to England and so generation had to be cut. He also confirmed the National Grid spent £280m balancing supply and demand.

A spokesman for the Department for Energy and Climate Change (DECC), described the incident as "unusual" and said more electrical storage was needed. 

He added: "In future we need greater electrical energy storage facilities and greater interconnection with our EU neighbours so that excess energy supplies can be sold or bought where required."

A Scottish government spokesman said electricity generated by renewables accounts for 27.4%  electric use in Scotland.