Small wind turbines are considered a complement to photovoltaics because wind can also generate electricity in winter and at night. Many projects start with this expectation, yet the actual yield often remains low. The reason is usually the physics and wind conditions at the site, not regulations or theoretical figures. (ingenieur: 09.02.26)
Small wind turbines – rated power sounds good, but the capacity factor is what matters
A 5 kW nameplate looks like a 5 kW photovoltaic system, but the logic doesn’t add up. Photovoltaics operate with relatively stable irradiance statistics, while wind fluctuates significantly from place to place. Furthermore, wind power increases with the cube of the wind speed.
A site with an average wind speed of 5 m/s produces more than four times the output of a site with 3 m/s, which is why the annual output can quickly change. In practice, the capacity factor is what matters—that is, the actual energy output relative to the theoretical full load—and this is often only 5 to 15 percent for small winds. With a rated output of 5 kW and an average wind speed of around 4 m/s, typical operating outputs are frequently only 400 to 800 watts. In inland Germany, this often results in approximately 1,500 to 3,000 kWh per year. However, small wind turbines only achieve these values at suitable locations.
Turbulence is the yield killer in residential areas
The limiting factor is rarely the rotor itself, but rather the condition of the airflow. In residential areas, highly turbulent air currents are created by houses, trees, hedges, and other obstacles. Turbulence intensities of 20 to 40 percent are realistic there, while good open-field locations often have turbulence intensities below 10 percent.
High turbulence reduces the power coefficient, increases load changes, and accelerates wear. It also leads to more frequent noise conflicts. As a result, turbines shut down more often, and yields plummet. For small wind turbines, turbulence is therefore not a minor issue, but the primary risk.
Mounting Location – Roof is Typical, Mast Usually Better
Many turbines are installed on roofs because it’s the most practical solution and because a freestanding mast often appears more difficult to mount. However, roof mounting is frequently aerodynamically unfavorable because roof edges and structures can create turbulence and transmit structure-borne noise into the building. Roof mounting can work if the turbine is positioned well above the ridge and the prevailing wind direction remains unobstructed, but this is not the norm.
From an engineering perspective, freestanding masts with sufficient clearance from obstacles often deliver better results. The crucial factor remains the hub height, as even small increases in height measurably increase wind speed. A difference of 10 m height at approximately 3.5 m/s compared to 20 m height at approximately 4.5 m/s can mean almost a doubling of energy.
Without wind field measurements, planning remains speculation
Wind atlases and online calculators provide only rough guidance because they use average values at altitudes that are more relevant for high winds. At the actual site, obstacles significantly alter the wind field, and the yield can quickly fluctuate by a factor of two. Reputable guidelines therefore recommend wind field measurements over six to twelve months at the intended hub height, including gusts and turbulence.
Only then can it be assessed whether the site even falls into a viable class, because IEC 61400-2 sets a lower limit of 6.0 m/s for Class IV, which many inland locations fall below. Economically, small-scale wind power often remains limited because, with an investment of €30,000 and an annual yield of 2,000 kWh, the levelized cost of electricity (LCOE) is roughly €0.75 per kWh. Small-scale wind power is more likely to be viable in rural areas or as part of a hybrid system with PV, storage, and load management, because wind power can then supplement the existing system over time.