The research will fuel criticism of wind farms which are already unpopular among many local communities who say they blight the landscape, generate noise and may disrupt wildlife.
But the government which plans to increase the number of turbines from the current 3,873 to 10,000 within the next decade, insists they are essential to fight climate change.
Predictions: Professor Gordon Hughes, an economist at Edinburgh University and former energy advisor to the World Bank
Ministers are currently drawing up plans to compensate local communities affected by them with investment – which have critics have dismissed as bribery.
Prof Hughes’ study of 280 wind farms in Britain and more than 800 in Denmark from 2000 to 2011, found the larger wind farms typical in Britain are less effective than smaller ones.
In Denmark, where wind power has been used for longer, the decline in output was less dramatic, which he said could be down to their smaller size and possibly better maintenance.
For onshore wind, the monthly ‘load factor’ of turbines – a measure of how much electricity they generate as a percentage of how much they could produce if on at full power all the time - dropped from a high of 24 per cent in the first year after construction, to just 11 per cent after 15 years.
For offshore wind –examined only in Denmark where it has been used for longer - it declined even more dramatically from over 40 per cent at the start, to just 15 per cent after ten years.
He believes they become uneconomic after around 12 years. The decline in output was put down to wear and tear of the blades, and more frequent breakdowns for older turbines.
His report for the anti-wind farm charity the Renewable Energy Foundation (KEEP), noted: ‘Onshore wind turbines represent a relatively mature technology, which ought to have achieved a satisfactory level of reliability in operation as plants age. Unfortunately detailed analysis of the relationship between age and performance gives a rather different picture…’
Prof Hughes told the Sunday Telegraph the trend for larger wind turbines in Britain, which reach up to 400feet tall in some areas increased wear and tear.