Once they are built, wind turbines produce "clean" electricity with no carbon dioxide emissions, whereas coal or gas-burning power stations pump out emissions.
The British Wind Energy Association (BWEA) has for years applied a formula to show how many tonnes of emissions wind turbines save per unit of electricity, compared to conventional power.
Now the Advertising Standards Authority has told the association to re-jig its formula, in effect halving the amount of emission saved.
So it means you now need twice as many wind turbines to generate the same savings in emissions.
The new formula is now given on the BWEA website.