Rated wind speed
The lowest wind speed at which a turbine reaches its nameplate (rated) power output, typically 10 to 13 m/s for modern utility-scale turbines.
Between cut-in and rated speed a turbine operates in "region 2": the controller adjusts rotor speed to hold the optimal tip-speed ratio and extract as much power as possible, so output rises roughly with the cube of wind speed. Above rated speed ("region 3") output is capped at nameplate power by pitch control, shedding the surplus energy in the wind.
Rated speed follows from the balance between generator size and rotor size, often expressed as specific power (rated W per m² of swept area):
- High specific power (400–450 W/m²): rated near 12.5–13.5 m/s, suited to IEC Class I high-wind sites
- Low specific power (200–300 W/m²): rated near 9.5–11 m/s, the dominant onshore trend since about 2015
- Offshore (300–400 W/m²): rated around 10.5–12 m/s
Lower rated speed means a turbine hits full output more often, raising the capacity factor and smoothing production, at the cost of a larger and heavier rotor per MW. Berkeley Lab analysis of US projects attributes much of the rise in onshore capacity factors over the past decade to this shift toward lower specific power.
Related terms
All terms →- Power curve
The relationship between hub-height wind speed and a turbine's electrical output, used with a wind speed distribution to estimate annual energy production.
- Cut-in wind speed
The lowest wind speed, measured at hub height, at which a wind turbine starts producing net electrical power, typically 2.5 to 4 m/s for modern machines.
- Cut-out wind speed
The wind speed at which a turbine shuts down or ramps down to protect itself from excessive loads, typically 25 m/s (10-minute mean) for utility-scale machines.
- Swept area
The area of the circle traced by a turbine's rotating blades; wind turbine power output is directly proportional to it, and it grows with the square of rotor diameter.
- Capacity factor
The ratio of the energy a power plant actually produces over a period to the energy it would produce running at full nameplate capacity for the whole period.
- Pitch control
Rotation of wind turbine blades about their long axis to change the angle of attack, used to regulate power above rated wind speed, start up, and stop the rotor.