A wind turbine captures some of the energy in the wind. The blades of a wind turbine are shaped like propellers that are driven by the wind. There are usually three blades, which together are called a rotor. They capture energy from the movement of the wind, as opposed to a motor that uses energy to create movement.
The rotor is mounted on a shaft that is connected to a generator. It converts the spinning motion into electricity. Since the rotor spins quite slowly, a gearbox is often placed between the shaft and the generator to achieve a consistent speed. More modern wind turbines often have generators directly on the rotor shaft, which provide power even at low speeds. Wind turbines are connected to the electricity grid and supply power as long as they spin.
A wind turbine cannot capture all of the wind's energy. The theoretical limit is 59 percent. The efficiency – how much of the wind's energy is utilized – is close to this limit for modern wind turbines; between 40 and 50 percent efficiency. The larger the wind turbines, the higher the efficiency can be. The energy yield in weak winds is low, but increases rapidly when the wind picks up. If the wind speed is doubled, the energy yield increases by eight times. Wind turbines are built to be most efficient at a normal wind (rated wind), often around 12 to 14 meters per second. When a storm blows, the wind turbine is shut down to protect it from excessive stress.
Another measure used is the utilization rate or capacity factor. It is about how much energy an entire wind farm generates, in relation to if it delivered maximum all the time. For onshore wind power in Sweden, the capacity factor is 35–40 percent, but it is increasing as new technology is developed.