Why is a wind turbine's output less than available wind power?

A wind turbine's blades turn because moving air exerts aerodynamic forces with a turning component. Their shaft drives a generator, transferring some of the wind's kinetic energy toward electricity. Air leaving the rotor still moves, so the available energy passing through the swept area is larger than the energy extracted.

Available wind power is the energy passing through the swept area each second. Two effects explain its cubic dependence on wind speed. Each kilogram has kinetic energy proportional to speed squared, while faster wind also carries more kilograms through the area each second. Together these give Pwind = ½ρAv³, with density ρ, area A and speed v.

For the pictured uniform wind, density is 1.2 kilograms per cubic meter, area is 10 square meters and speed is 5 meters per second. The available power is 750 watts. Doubling speed at fixed density and area multiplies this incoming energy rate by eight. It does not automatically multiply electrical output by eight.

The illustration supplies a capture ceiling of 40 percent and a further conversion efficiency of 90 percent. Those separate factors give 750 × 0.4 × 0.9 = 270 watts. Capture describes energy transferred from the airflow; conversion describes what then reaches the electrical output. Neither factor is a universal value for turbines.

The 1,000-watt electrical rating caps this model's output. At 10 meters per second, regulation must reduce capture below the unconstrained calculation; unused energy can remain in the airflow. Wind variation, operating limits and control also shape real performance, so these snapshots explain power conversion without determining annual energy production.