and if it is too high (more than about 25m/s) the turbines will have to be shut down to avoid damage. When this happens other power sources must have the capacity to meet demand, Three reports on the wind variability in the United Kingdom issued in 2009, generally agree that variability of the wind does not make the grid unmanageable; and the additional costs, which are modest, can be quantified. For wind power market penetration of up to 20% studies in the UK show a cost of £3-5/MWh. In the United Kingdom, demand for electricity is higher in winter than in summer and so are wind speeds. While the output from a single turbine can vary greatly and rapidly as local wind speeds vary, as more turbines are connected over larger and larger areas the average power output becomes less variable. Studies by Graham Sinden suggest that, in practice, the variations in thousands of wind turbines, spread out over several different sites and wind regimes, are smoothed, rather than intermittent. As the distance between sites increases, the correlation between wind speeds measured at those sites, decreases. The 2021 United Kingdom natural gas supplier crisis increased electricity prices, which were further worsened by rising demand amidst a lack of wind. During storms, power prices have occasionally become zero or even negative. Constraint payments The development of the GB grid was characterised by the close proximity of major sources and demand for electricity. Since wind farms tend to be sited far from centres of demand, transmission capacity can be inadequate to deliver electricity to users, particularly when wind speeds are high. When the grid cannot deliver electricity generated, wind farm operators are paid to switch off. It is normally necessary to pay another generator – normally a gas-fired power station – on the other side of the constraint to switch on as well, to ensure that demand is met. These two incentives are referred to as "constraint payments" or curtailment, and they are one source of criticism of the use of wind power and its implementation; in 2011 it was estimated that nearly £10million in constraint payments would be received, representing ten times the value of the potential lost electricity generation. Wind farm constraint payments have increased substantially year on year, £224million, out of a total of £409million in 2020–21. In addition, £582million was spent rebalancing the system afterwards, mainly to gas-fired