Wind turbines larger than 6 MW are becoming common, particularly for offshore installations, where consolidating several smaller wind turbines into one large version reduces manufacturing, installation, and sustainment costs in accordance with economies of scale.
The overall reliability of the wind turbine is mostly determined by the mechanical gearbox, which converts the practical low speed of the wind-turbine rotor propeller hub (e.g., 5 to 20 RPM) to the variable high speed of a conventional electric generator system (e.g., two pole-pair, 60 Hz, 450 to 1800 RPM). In turn, large wind turbines are eliminating the gearbox (and its compounded inefficiency, complexity, size, and cost) with low-speed, directly driven electric generator systems. However, as physics dictates, low-speed electric generator systems are necessarily large in diameter (and heavy).