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A rather dull term to describe the large amounts of money paid, mainly to energy companies running wind farms, to switch off or slow down wind turbines when the potential supply is greater than demand, or the transmission network does not have the capacity to take it where it is wanted. These payments ultimately come from the consumers. Most of these constraint payments are paid to the operators of Scottish wind farms. Increasingly more electricity is being generated in Scotland than can be used there, and the grid interconnections between Scotland and England are insufficient to take the excess electricity which is generated, usually at times of, often unexpectedly, high winds and low Scottish demand. This information is in the public domain at Link and detailed figures can be obtained at Link.

Some examples of total payments for 2011-2013 are :-
Fallago Rig NT6063 - £2.1m [2013 only]
Crystal Rig II NT6767 – £2.5m
Hadyard NX2395 - £3.17m
Whitelee NS5443 - £3.6m
Farr NH7329 - £5.0m
Clyde North and South NS9911-NS9826 – £1.9m

Payments on single days have amounted to £1.8m [3 Aug. 2013]; £0.96m [2 Aug. 2013]; £1.0m [3 June 2013]; £1.15m [29 April 2013]; £1.67m [11 Sep. 2013].
On 5-6 April 2011 almost £1.67 was paid to the operators of 6 Scottish wind farms.
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£8.7 million paid to 27 wind farms in March 2014 not to generate power [Daily Telegraph, 5 April 2014].

Offshore generation also benefits from huge subsidies - apparently contracts for offshore wind farms that start running in 2014-16 will offer £155 for each MWh of electricity generated - about three times the market price [Daily Telegraph, 10 Sept. 2013] - some subsidy!

A wind turbine at Welsh Government Offices in Aberystwyth cost £48000. In the first 18 months it produced an average of £5.28-worth of electricity/month. There were mechanical problems and, after improvement, it is now generating £8.84-worth/month. So another 451 years and it will have paid for itself [Daily Telegraph, 8 November 2013]. Thanks to Rudi Winter for this link and comment:- You can see the turbine on Streetview: Link It's a tiny ornamental turbine which is about as functional as a windchime, sited inappropriately .
This useful and interesting site Link gives detailed information about the UK National Grid Status, current and past usage and demand for power, and the means by which it is produced, updated every few minutes.
Wind farm operators were paid £7.6million compensation in 2012-13, and £27.9million so far in 2013-14, as more wind farms come on line and the National Grid is unable to cope with the amount of energy being produced. It is suggested that heavy industry may be paid to operate at night, to make use of this oversupply [Daily Telegraph, 8 Jan. 2014].
'£3m bill for turning wind turbines off on a blustery day' - Daily Telegraph headline, 13 August 2014. That was the tail end of Hurricane Bertha - £2.8m paid to owners to shut down turbines, and another £1.1m paid to power stations to make up the shortfall resulting from the shutdown.
The Daily Telegraph, 3 January 2015, headline:- 'Scotland has enough windfarms'. The article cites a WWF Scotland report Link that wind power had generated enough power to supply 98% of the country's households in 2014. It's a bit unclear about industrial needs! £53.2 million was paid to wind farms not to generate in 2014 - including £12 million to Whitelee, East Renfrewshire, the UK's largest wind farm with 215 turbines. £638000 was paid to Whitelee on New Year's Day alone.
Apparently on 19 Jan. 2016, when it was -7C in Somerset, and windless, Britain's 5500 wind turbines contributed 66MW, which was 0.1% of the total output [C. Booker, Sunday Telegraph, 24 Jan. 2016].

Robert Mendick [Daily Telegraph 6 Jan. 2018], quoting the Renewable Energy Foundation Link , reports that in 2017 wind farms were paid £108 million not to produce electricity, compared to £6 million for 2016. Constraint payments for the last 5 years totalled £367 million. Current payment rates are £70/MWh to switch off, compared to a typical £49/MWh consumer subsidy when generating. They need to be switched off because the grid is unable to cope with their output. The money is paid out to wind farmers by the National Grid, but is ultimately charged to consumers.