Wind power continues to grow rapidly in the United States, due to the production tax credit, various state-level policies, and improvements in the cost and performance of wind power technologies. The falling cost of wind energy has been attractive to utility, corporate, and other power purchasers.

The new 2017 Wind Technologies Market Report, prepared by Berkeley Lab and released by the U.S. Department of Energy, documents trends in wind power cost, performance, prices and technology choices.

One technology change has been the increasing size of wind turbines: the average capacity of a wind turbine installed last year is more than three times bigger than twenty years ago. Bigger machines — larger generators, taller towers and longer blades — are a significant factor in the declining electricity cost and improving performance of wind power projects. And wind turbine size is expected to continue to grow. DOE is investigating ways to overcome design and logistical hurdles to even larger rotors.

But while the move to bigger turbines has been well-noted, one important trend is getting less attention, the fall in "specific power."

Specific power measures the ratio of the generator size to the rotor size. Generators are measured in Watts (W) of nameplate capacity. Rotors are measured, in this case, by the area they sweep (the "swept area"), measured in square meters (m²). Machines with low specific power have a larger rotor size relative to generator size, thus fewer Watts per square meter (W/m²).

Since bigger rotors catch more wind, they transfer more energy to the generator. Low specific power wind turbines are not always optimal. In especially high and turbulent winds, such turbines may experience a lot of stress, and larger rotors come at a cost.

Such turbines do better in lower wind speeds. In fact, low specific power turbines were originally designed to catch the breezes in lower wind speed sites, whether in parts of the Great Lakes, Mid-Atlantic or Southeastern United States. But, in an ongoing but notable trend, wind power developers have been installing them across the country, even at sites with relatively high wind speeds.

Why is that?

The most important reason so far is that such turbines provide greater bang-for-the-buck than standard turbines.

The greater revenue from power production and tax incentives more than offsets the higher up-front capital costs and greater wear and tear. Low specific power turbines have helped reduce the cost of wind energy — important in a market currently awash with inexpensive electricity, whether from gas, coal, solar or wind.