A wind farm 'life cycle assessor' adds up the energy needed to mine, refine, process and transport the materials in a turbine and adds the energy used to maintain and decommission a wind farm at the end of its life. Then they compare this with the energy produced.

A 2010 University of Vermont study brought together data for 119 wind farms and found that, on average, they produce almost 20 times the energy used to build them; twice as good as coal (Kubiszewski et al, 2010).

For the 69 that also reported carbon emissions, the average was 25 grams of CO2 for each unit of electricity generated; 98% savings versus an average coal power station.

Running backup power stations reduces savings, but the savings are still big

When the weather calms other power stations have to take up the load. Turning a generator up and down is inefficient: many of us experience this when we pay for the difference in mpg between stop-start city driving and smooth highway driving.

Valentino et al, 2012 have now calculated this effect based on real weather data and wind forecasts for 15 sites in Illinois. The inefficiency from turning power stations on and off takes a bite out of the savings that wind power brings, but since the most polluting power stations get turned off first the savings are still big.

If Illinois got 10% of its electricity from wind, the savings in CO2 would be about 11%. The savings are bigger than 10% because about a quarter of Illinios' power is low carbon nuclear and this wouldn't be turned off.

As more wind turbines are added, they still make savings but if Illinois got 40% of its electricity from wind, its CO2 emissions would drop only 33% as nuclear reactors scale back slightly.