The calculated or measured energy delivered by a wind turbine generator (WTG) or small wind turbine (SWT).

An electrical current whose direction reverses periodically, usually many times per second. The World households electrical systems use AC current rated at 120 V and 60 cycles per second (North America and others) or 230 V and 50 Hz, (Europe and others).

A unit for quantity of electricity supplied or used at some fixed (usually dc) voltage. Obtained by integrating current flow in amperes over time in hours. Multiplying by the voltage gives the energy exchanged. Used as a measure of battery capacity.

calculated as the average of short periods, e.g. 10 minute averages.

The mean wind speed over a specified period of time. For example, for assessing a horizontal axis Wind Turbine power curve, (i) the unperturbed wind speed is measured at hub height and then averaged within successive periods of 10 minutes, (ii) the wind turbine power is similarly averaged within concurrent periods.

The calculated or measured energy delivered by a wind turbine generator (WTG) or small wind turbine (SWT).

A theoretical maximum amount of energy that a perfect wind turbine can extract from the wind approaching its rotor, which Betz calculated as 59% of wind kinetic energy. In practice, efficient wind turbines have efficiencies about 70% of this Betz criterion.

A device capable of reducing or stopping the rotor speed.

The budgeted energy for a wind plant, usually defined per month.

The percent of actual energy for a wind turbine versus the rated maximum power over a specific time period.

Actual Energy / (Actual Energy + Commercial Lost Energy)

The percent of time that data is available for a turbine or meteorological station.

An electrical current flowing in one direction only (i.e. not AC), defined as from positive to negative voltage.. Typical sources of DC current are batteries of electrochemical cells, photovoltaic cells, and rectifiers of AC. DC current has to be transformed to AC at mains voltage through an inverter for most conventional household appliances.

A wind turbine whose rotor shaft is directly connected to the electrical generator shaft, i.e. without an intermediate gearbox.

The direction to which the wind is blowing, e.g. the leeside is downwind.

The percent of time that a turbine is available to produce energy due to planned and unplanned outages that are within the plant's control. Excludes non-operating conditions (i.e., high wind shutdown, untwisting, high/low ambient temperature) and outages outside of the plant's control, such as force majeure, curtailment, utility outages, and wake management. Base Hours is the total hours for the time period less any missing data hours. EAF = 100 * (1 - (Plant Planned Outage Hours + Plant Unplanned Outage Hours) / Base Hours)

The percent of time that a turbine is unavailable to produce energy due to forced outage conditions that are within the plant's control. Excludes planned outages, non-operating conditions (i.e., high wind shutdown, untwisting, high/low ambient temperature) and outages outside of the plant's control, such as force majeure, curtailment, utility outages, and wake management. Base Hours is the total hours for the time period less any missing data hours. EFOR = 100 * Plant Forced Outage Hours / Base Hours

The expected energy based on actual wind speed and the theoretical power curve, adjusted for actual air density. When using the turbine anemometer wind speed, free-stream adjustments shall be applied, as necessary.

Condition of a wind turbine that is rotating a little over its rated rpm and not generating shaft power. This is only achieved with performing controlled feathering or stalling regulation systems.

A mechanical assembly, with an input and an output shaft, to house gears. The gearbox is positioned between the main shaft and the electrical generator, thus matching the rotor speed (say 60 rpm) to the recommended speed of the generator (say 1 500 rpm).

Actual Energy / (Actual Energy + Gross Lost Energy)

The central fixture of a HAWT rotor, to which the blades of a wind turbine are attached. If the blades are firmly bolted to the hub; the rotor is called 'fixed pitch'. However, blades can have a degree of freedom to turn (pitch) along their longitudinal axis by interposing of a rotating device, ball bearings, slewing ring or turntables. The rotor is then called 'variable pitch'.

Condition of a wind turbine that is rotating slowly and not producing power.

An electronic device that converts direct current (DC) to alternating current (AC) at the required frequency (e.g.50Hz for grid electricity in Europe.)

1000 Watts, a measure of power (energy per unit time).

1000 Watts for one hour; energy unit (energy per unit time, multiplied by time = energy). The energy dissipated by using one kW in one hour.

The expected energy lost while a turbine is unavailable to produce energy and there is sufficient wind for generation.

The shaft connecting the rotor hub to the gearbox or, with direct drive, to the generator.

The body (including frame and cover) that contains the main bearings, brakes, gearbox, generator, pitch controllers, hydraulic equipment etc of a wind turbine.

The percent of actual energy produced at the turbine divided by the expected energy for the turbine when the turbine is online.

The percent of actual energy delivered to the interconnect divided by expected energy.

The ratio of the power extracted by a wind turbine to the power available in the wind stream ahead of the rotor area at a specified wind speed.

A graph showing a wind turbine's power output as a function of wind speed.

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

The main rotating drive mechanism of a wind turbine that includes the blades and blade hub.

The outer diameter of the rotor. That is approximately twice the blade length.

A device for making electrical connection between stationary and rotating parts, e.g. as in generators or for connecting monitoring sensors to a rotor or for connecting mains cables from a rotating nacelle to within a tower. Usually it includes a bronze or other conducting alloy circular component over which carbon conducting brushes move with minimal friction.

The area swept by the turbine rotor, for common HAWT, A=pR2 where R is the radius of the rotor.

According to the IEC 61400 standard, SWTs are wind turbines of swept area less than 200 m2, i.e. HAWT less than 16 m in diameter. However, this definition is not universal.

The percent of actual energy produced at the turbine divided by the expected energy for the turbine.

Change in wind speed and direction over periods of seconds. Turbulence is an intrinsic property of wind that is increased by obstacles.

Ratio of the standard deviation of the wind speed to the mean wind speed, as determined from the same set of measured data of wind speed for a specified period of time.

On the same side as the direction from which the wind is blowing => windward.

Vertical axis wind turbine.

The increase of wind speed above the ground as a function of height. A logarithmic expression is sometimes used to characterize wind shear.

The movement of the tower top (nacelle) of a HAWT that turns the turbine rotor with respect to the oncoming wind.