About 500,000 solar panels were installed every day last year as a record-shattering surge in green electricity saw renewables overtake coal as the world’s largest source of power capacity. Two wind turbines went up every hour in countries such as China, according to International Energy Agency officials who have sharply upgraded their forecasts of how fast renewable energy sources will keep growing. “We are witnessing a transformation of global power markets led by renewables,” said Fatih Birol, executive director of the global energy advisory agency. Part of the growth was caused by falls in the cost of solar and onshore wind power that Mr Birol said would have been “unthinkable” only five years ago. Average global generation costs for new onshore wind farms fell by an estimated 30 per cent between 2010 and 2015 while those for big solar panel plants fell by an even steeper two-thirds, an IEA report published on Tuesday showed.