Renewables UK, the trade association for the UK renewable industry, recently announced that total installed wind generating capacity had exceeded 6,000 MW, “enough to supply electricity to 3,354,893 homes”.

We’ll overlook the absurdly precise figure, which is designed to impress the innumerate. Let’s just say more than 3.3 million homes.

Since there are around 26 million households in the UK, that represents more than 12 per cent of them. The figure is based on an average annual domestic consumption of 4.7 MWh and a 30 per cent average capacity factor for the turbines.

So how much electricity did wind turbines generate yesterday, when demand peaked at 58,242 GW, the highest of the winter so far? Data issued daily by the National Grid on the NETA website shows that wind generated just 1.3 per cent of the total electricity consumed between 11.30 am yesterday and the same time today, or 14,926 MWh out of a total of 1,177,929 MWh.

So on one of the coldest days of the year, when consumers really need electricity, wind turbines produced enough for (by my calculations) just over 330,000 homes – a tenth of the figure cited by Renewables UK.

We’re in the middle of a very cold but settled spell, without a lot of wind, at least in southern England. And, to be fair to wind power, not all the 6 GW of installed capacity is connected to the National Grid, which lists just over 4 GW of wind capacity. The gap is made up of small local wind generators which will have made a contribution, albeit modest and unmeasured, to meeting demand.

However, the figures do emphasise the issue of intermittency. Installing green generating capacity that isn’t available when you most need it means that continuing non-green capacity must be provided, too. Over an entire year, Renewables UK’s figures are doubtless about right, though a 30 per cent capacity factor is a slight exaggeration – data cited by the Renewable Energy Foundation suggest that UK windfarms of over 1MW installed capacity have never exceeded a 28 per cent capacity factor on an annual basis and in 2010 achieved only 24 per cent.

However, the current cold snap is less extreme that that of 7 December 2010, when demand peaked at 5.30 in the afternoon at over 60 GW and the wind turbines then installed (5.2 GW in total) generated just 300 MW of it. “Such figures confirm theoretical arguments that regardless of the size of the wind fleet the United Kingdom will never be able to reduce its conventional generation fleet below peak load plus a margin of approximately 10 per cent” said the Renewable Energy Foundation at the time.