A wind turbine is a machine for slowing down air. Every kilowatt-hour it delivers started as kinetic energy in the wind upstream. Each design decision (blade length, tower height, the twist of the airfoil, how the rotor is steered and braked) aims to extract as much of that energy as possible, as cheaply as possible, for 25–30 years without breaking. This guide covers the physics, the main components and the control system, and shows how to estimate what a given turbine will produce at a given site.
The energy in moving air
The power carried by wind through an area facing it is:
Two terms dominate. Swept area grows with the square of blade length, so doubling the rotor diameter captures four times the energy. Wind speed enters as a cube: a site averaging 7 m/s carries about 2.4 times the power of one averaging 5.25 m/s, not 1.33 times. The cube law explains why siting is so sensitive, why towers keep getting taller, and why a small error in measured wind speed turns into a large error in predicted energy.
Air density matters less, but you can’t ignore it. It drops by roughly 1% per 100 m of elevation and rises in cold weather, so at the same wind speed a turbine produces slightly more on a cold winter day than on a hot summer one.
The Betz limit: why you cannot capture it all
A turbine that took all the kinetic energy would stop the air dead behind it and block the flow. One that took none would let the air pass straight through. The best result lies in between. In 1920 Albert Betz used a simple momentum analysis to show that an ideal open rotor can extract at most 16/27 of the energy, about 59.3%. It gets there when the wind far downstream has slowed to one third of its upstream speed.
The ratio of actual rotor power to the power in the wind is the power coefficient, C_p. Real three-bladed rotors peak at a C_p of roughly 0.45–0.50, about 80% of the Betz limit. The rest goes to blade-tip vortices, drag on the airfoils, wake rotation and the hub. Gearbox, generator and converter losses take another 5–10% combined, leaving an overall wind-to-grid efficiency of around 40–45% at the best operating point.
How the blades extract energy
Modern blades work like aircraft wings, not sails. Air flowing over the curved airfoil produces lift at right angles to the apparent wind the blade sees, plus a smaller drag force along it. Because the blade is moving, the apparent wind combines the real wind with the blade’s own motion. At the tip of a large rotor the blade travels at 70–90 m/s against a wind of perhaps 10 m/s, so the apparent wind arrives almost edge-on.
This shapes blade design in two ways:
- Twist. The tip moves much faster than the root, so the angle of the apparent wind changes along the blade. Blades are therefore twisted, steeply near the root and almost flat at the tip, so that every section meets the air at its best angle of attack.
- Tip speed ratio (TSR). Tip speed divided by wind speed. Three-bladed rotors hit their peak C_p at a TSR of about 7–9. In light and moderate winds, a variable-speed turbine changes rotor speed with the wind to stay close to that value.
Blades have grown enormously, from about 20 m in the late 1990s to over 100 m on today’s largest offshore machines. NREL’s IEA 15 MW reference turbine (2020) has a 240 m rotor diameter. That sweeps about 45,000 m², roughly six football pitches.
Inside the nacelle: drivetrain options
The rotor of a large turbine turns slowly, typically 5–15 revolutions per minute, far too slow for a conventional generator. Manufacturers solve this in two main ways, plus a hybrid:
| Drivetrain | How it works | Pros | Cons |
|---|---|---|---|
| Geared (DFIG or full converter) | 3-stage gearbox steps up to 1,000–1,800 rpm | Compact, cheaper generator | Gearbox maintenance and failures |
| Direct drive | Large-diameter permanent-magnet generator turns at rotor speed | Fewer moving parts, high reliability | Heavy, uses rare-earth magnets |
| Medium speed (hybrid) | 1–2 stage gearbox, mid-speed PM generator | Balance of size and reliability | Still has a gearbox |
Most turbines today put a power electronic converter between generator and grid. It separates rotor speed from the fixed grid frequency, which allows variable-speed operation, and lets the turbine support the grid with reactive power and fault ride-through.
A few other systems complete the picture. The yaw drive turns the nacelle into the wind, guided by a wind vane and anemometer on the nacelle roof. The pitch system rotates each blade about its long axis. A mechanical brake holds the rotor still during maintenance.
The power curve and turbine control
Every turbine has a certified power curve: electrical output against hub-height wind speed, measured to the IEC 61400-12-1 standard. It has four regions:
- Below cut-in (0–3 m/s): too little energy to overcome losses, so the rotor idles.
- Partial load (3–11 m/s): the controller varies rotor speed to keep the TSR on target and C_p near its maximum. Output climbs roughly with the cube of wind speed.
- Rated power (≈11–25 m/s): the generator runs at full output. The pitch system feathers the blades to spill excess energy and hold power steady.
- Cut-out (above ≈25 m/s): the turbine pitches its blades fully and stops to protect itself. Many modern machines use a gradual “storm control” ramp in place of an abrupt stop.
The rated wind speed is a design choice. Pair a large rotor with a relatively small generator, giving a low specific power (W/m² of swept area), and the turbine reaches rated output at lower wind speeds and runs at full power more often. Onshore turbines for low-wind sites have moved towards 200–300 W/m². This raises the capacity factor but costs more rotor material.
Worked example: annual energy of a 5 MW onshore turbine
Take a modern onshore turbine with a 160 m rotor, rated at 5 MW (specific power ≈ 250 W/m²), on a site with a mean wind speed of 7.5 m/s at its 140 m hub height.
First, a quick upper-bound check at the mean wind speed. Swept area = π × 80² ≈ 20,100 m². Power in the wind at 7.5 m/s = 0.5 × 1.225 × 20,100 × 7.5³ ≈ 5.2 MW. At C_p × η ≈ 0.42, that is about 2.2 MW.
The mean wind speed alone underestimates the energy, though, because the cube law gives strong winds a heavy weight. Wind speeds follow a skewed distribution, usually modelled as a Weibull distribution with shape factor k ≈ 2. Integrating the power curve over it gives:
| Quantity | Value |
|---|---|
| Gross annual energy (power curve × Weibull, k = 2) | ≈ 18–20 GWh |
| Losses: wake, availability, electrical, icing, curtailment (10–20%) | −2 to −4 GWh |
| Net annual energy | ≈ 15–18 GWh |
| Capacity factor = net energy ÷ (5 MW × 8,760 h) | ≈ 35–40% |
Our wind turbine calculator does this integration with real turbine power curves, a Weibull fit to satellite-derived wind data at your location and a loss budget you can adjust. If all you have is the wind speed at 10 m, first extrapolate it to hub height with the wind speed at height calculator.
Why turbines keep getting bigger
Bigger turbines cut the cost per kWh on several fronts. Taller towers reach faster, steadier wind. Longer blades capture energy in proportion to the square of their length, while many costs (foundations, cabling, permits, maintenance visits) are paid per turbine, not per MW. IRENA’s Renewable Power Generation Costs in 2023 reports that the global weighted-average LCOE of new onshore wind fell by around 70% between 2010 and 2023, driven largely by larger, taller turbines and higher capacity factors.
The limits are logistical and structural: moving 80+ m blades by road, finding cranes for hub heights of 150+ m, and blade mass that grows faster than length. At sea, ships replace trucks and those limits relax, so the biggest machines go offshore. See offshore wind explained.
Key metrics to compare turbines and sites
| Metric | Typical onshore | Typical offshore |
|---|---|---|
| Rated power (new installations) | 3–7 MW | 8–15 MW |
| Rotor diameter | 120–175 m | 160–250 m |
| Hub height | 100–170 m | 100–150 m |
| Capacity factor | 25–45% | 40–55% |
| Design life | 25–30 years | 25–30+ years |
To see how a turbine’s real output compares with its nameplate, read what is capacity factor. For how that becomes a cost per kWh, see LCOE explained and the LCOE calculator. The rankings and the world energy map show which countries get the largest share of their electricity from wind.
Calculators for this topic
Frequently asked questions
How much electricity does one wind turbine produce?
What is the Betz limit?
Why do wind turbines have three blades?
At what wind speed do turbines start and stop?
Why are wind turbines so tall?
Do wind turbines work when the wind is not blowing?
Sources & references
- Betz, A. (1920). Das Maximum der theoretisch möglichen Ausnützung des Windes durch Windmotoren. Zeitschrift für das gesamte Turbinenwesen
- Burton, T., Jenkins, N., Sharpe, D., Bossanyi, E. (2011). Wind Energy Handbook, 2nd ed. Wiley
- U.S. DOE — How Do Wind Turbines Work?
- NREL — Definition of the IEA 15-Megawatt Offshore Reference Wind Turbine (NREL/TP-5000-75698)
- IRENA — Renewable Power Generation Costs in 2023
Figures in this guide are typical ranges drawn from the sources above; your own numbers depend on location, prices and equipment. Use the linked calculators for site-specific estimates.