Glossary of Terms
Wind Power Services LLC has compiled a list of definitions for wind energy terms. If you did not find a definition for the term you were looking for, Please Contact Us and we will add it to our glossary.

AC
Alternating current, where the electrical current switches directions a fixed number of times per second (such as 60 times per second or 60 Hz)

Airfoil
The shape of the blade cross-section, which for most modern horizontal axis wind turbines, is designed to enhance the lift and improve turbine performance.

Ampere (A)
A measure of electric current; one A of current represents one coulomb of electrical charge moving past a specific point in one second (1 C/s = 1 A).

Ampere-hour
A unit of for the quantity of electricity obtained by integrating current flow in amperes over the time in hours for its flow; used as a measure of battery capacity.

Anemometer
A device to measure the wind speed.

Annual energy output (AEO)
The total energy produced by a wind turbine over the course of a year.

Average wind speed
The mean wind speed over a specified period of time.

Blades
The aerodynamic surface that catches the wind.

Brake
Various systems used to stop the rotor from turning.

Converter
See Inverter.

Cut-in wind speed
The wind speed at which a wind turbine begins to generate electricity.

Cut-out wind speed
The wind speed at which a wind turbine ceases to generate electricity.

DC
direct current

Density
Mass per unit of volume.

DOD
Depth of discharge

Downwind
On the opposite side from the direction from which the wind is blowing.

Furling
A passive protection for the turbine where typically the rotor folds either up or around the tail vane.

Generator set (genset)
Machine using an internal combustion engine (gasoline or diesel) and generator to produce AC or DC electricity.

Grid
The utility distribution system. The network that connects electricity generators to electricity users.

HAWT
Horizontal axis wind turbine.

Inverter
A device that converts direct current (DC) to alternating current (AC).

kW
Kilowatt, a measure of power for electrical current (1,000 watts).

kWh
Kilowatt-hour, a measure of energy equal to the use of one kilowatt in one hour.

MW
Megawatt, a measure of power (1,000,000 W or 1,000 kW).

Nacelle
The body/shell/casing of a propeller-type wind turbine, covering the gearbox, generator, blade hub, and other parts.

O&M Costs
Operation and maintenance costs.

Photovoltaics (PV)
The direct conversion of sunlight into electricity.

Power Coefficient
The ratio of the power extracted by a wind turbine to the power available in the wind stream.

Power curve
A chart showing a wind turbine's power output across a range of wind speeds.

PUC
Public Utility Commission, a state agency which regulates utilities. In some areas known as Public Service Commission (PSC).

Rated output capacity
The output power of a wind machine operating at the rated wind speed.

Rated wind speed
The lowest wind speed at which the rated output power of a wind turbine is produced.

Rotor
The rotating part of a wind turbine, including either the blades and blade assembly or the rotating portion of a generator.

Rotor diameter
The diameter of the circle swept by the rotor.

Rotor speed
The revolutions per minute of the wind turbine rotor.

Start-up wind speed
The wind speed at which a wind turbine rotor will begin to spin. See also cut-in wind speed.

Swept area
The area swept by the turbine rotor, A = π R 2 , where R is the radius of the rotor.

Tip speed ratio
The speed at the tip of the rotor blade as it moves through the air divided by the wind velocity. This is typically a design requirement for the turbine.

Turbulence
The changes in wind speed and direction, frequently caused by obstacles.

Upwind
On the same side as the direction from which the wind is blowing—windward.

VAWT
Vertical axis wind turbine.

W
watt, a measure of power for electrical current (1,000 watts = 1 kW).

Wind farm
A group of wind turbines, often owned and maintained by one company. Also known as a wind power plant.

Yaw
The movement of the tower top turbine that allows the turbine to stay into the wind.