It is fair to assume that the millions of viewers who tuned into EastEnders a week last Monday did not simultaneously give thanks for the wonder of wind. But it would have been right for them to applaud the spinning turbines which, we were later told by the National Grid, were providing a record 10% of all the electricity being used in Britain at the time.

The figure appeared to be a milestone. But it was followed by predictions that only 10% of all wind capacity will be available for use when the UK needs it most: in the depths of the coming winter. So which figure tells how close the UK is to achieving its targets on renewable energy?

The contrasting numbers explain why supporters of wind power – who say it will help combat climate change – and its opponents – who say it is an expensive and inefficient technology – always find something to lock horns about. While industry group RenewableUK will this week be celebrating the attainment of five gigawatts of wind capacity, detractors at the UK Independence Party (Ukip) are calling the claims a "tissue of lies".

The September record high for wind energy was reached because turbines were spinning at full pelt in windy weather while demand was lower than normal and other sources of power were not being used.

Critics argue that the unpredictability of wind turbines – which depend on the weather at any given time in a particular location – is nigh-on impossible for the grid to deal with. The grid says it assumes wind power is available at 30% of total capacity on average, but potentially at as little as 10%.

The latter figure compares with availability assumptions of 90% for coal or gas and even 100% for electricity provided through the interconnector link with France. Grid figures for the past three months show wind providing 3,000 gigawatt hours of power, compared with 48,000 for gas, 49,000 for coal and 13,000 for nuclear. These numbers seem to show turbines providing the grid with less than 3% of its power. However, it is thought that half of all wind farms supply customers direct – circumventing the grid – so the overall number may be nearer 6%.

This may seem low, but with a raft of new wind farms coming on stream – the world's biggest offshore facility opens officially off the Isle of Thanet on Thursday – capacity is growing fast. And when combined with new biomass plants (which burn crops) and old hydro schemes (using dams) the UK is close to meeting its 2010 target of generating 10% of electricity from green sources. The target was set eight years ago by the Labour government, and deemed a dream by many.

As recently as 2007, the UK was still less than halfway to meeting the energy target, and lagged behind most of Europe on renewable energy.

RenewableUK points out that despite continuing planning logjams and other hindrances, an entire gigawatt of capacity – enough to supply a city the size of Birmingham when the wind blows – was added last year. This is almost double the capacity of all the wind farms in the UK in 2002, and almost a fifth of the total current capacity, of 4.7 gigawatts. By the end of this month, another gigawatt of projects will have been built so far this year, half of them offshore.

Yet some remain implacably opposed to wind farms, particularly onshore ones. Next month, Godfrey Bloom, a Ukip MEP, is organising a seminar in London titled "Europe's Ill Wind – wind turbines and the tissue of lies". The party is opposed to the European-wide targets on renewables and wants a halt to large onshore wind farms and subsidies.

The 10% target is attainable not only because of the recent rapid addition of wind capacity, but also because the recession has reduced overall demand. Electricity consumption fell last year by 5%, mainly because of the downturn, and experts at accountant Ernst & Young doubt whether the 2010 target could be met if times were easier.

The challenge facing the industry and policymakers is the much tougher and legally binding EU 2020 renewables target. In the UK this requires 15% of all energy to come from renewable sources. Because this includes heating and transport – which are harder to "green" – experts say about a third of all electricity will have to be renewable to meet the target. Many energy executives are openly sceptical about this.

The coalition government said it was encouraged by the 10% figure. A spokesman for the Department of Energy and Climate Change said: "The pace at which industry is getting wind turbines up and getting other projects moving is encouraging, but there is still much more to do. The UK currently has the third-lowest proportion of renewables out of 27 EU states and we are determined that this improves."

That is just what Maria McCaffery, chief executive of RenewableUK, wants to hear from a government that claims it will be the greenest yet, even though industry critics are urging ministers to end the wind subsidy regime.

"Judging by these figures we are a hair's breadth from reaching our 2010 targets," she said, "mainly on the back of tremendous growth of wind energy. The message to take away is twofold: the target-based approach works, and 2020 targets are achievable."

Ben Warren, energy expert at Ernst & Young, is more cautious: "It's not time to rejoice quite yet: there is still some way to go to meet the 2020 targets. It gets more difficult to add each extra gigawatt of generation due to site availability and grid constraints."

Like EastEnders, the battle over the future of wind will go on and on.