As reviewed before, the increase of the size/power rating is due to the need to further reduce the wind energy cost. The analysis given by European wind industries (BWEA and DWIA) show that the cost of wind energy mainly includes the cost of wind turbine (64%), civil work (13%) and electrical infrastructure (8%). The turbine foundation cost, which amounts to a large part of the wind turbine cost and the total cost, is independent of the size of wind turbines. Moreover, the cost of electrical infrastructure also does not change with the wind turbine rating. Therefore, the cost of energy per unit of larger wind turbines is lower.
The Economics of Wind Energy— given by British Wind Energy Association
http://www.bwea.com/ref/econ.html
Size of Wind turbines— given by Danish Wind Industry Association