Gerard Wynn at Reuters reports on the performance of windfarms, both on and offshore, in the UK. It is not a pretty sight:
Garrad Hassan's report, "Offshore wind: Economies of scale, engineering resource and load factors", sought to "provide a supported opinion on the likely load factors for offshore wind power in the UK Round 1 and 2 offshore wind farms".
Government studies have since cited it as a reference.
The report calculated a range of load factors from 33 to 38 percent, for wind speeds ranging from 8.5 to 9.5 metres per second (See Chart 4).
Actual annual average load factors in fact only passed 35 percent for the first time last year, according to the Department of Energy and Climate Change's (DECC) "Digest of United Kingdom Energy Statistics 2012".
For previous years they were in the range 26-30 percent, or 28-35 percent depending on whether they are measured including or excluding turbines under construction.
More recent data show that load factors this year dipped back below the bottom end of the Garrad Hassan range.
The full article is here, and is well worth a look. It appears that "wake losses" i.e. losses from some turbines being in the wind shadow of others, are the problem.
Bigger windfarms required I guess.