Wind Energy for Homes: Is a Small Wind Turbine Right for You?

P = 0.5 × ρ × A × v³

• ρ — air density, about 1.225 kg/m³ for the International Standard Atmosphere at sea level and 15 °C. It is lower at altitude and in warm air.
• A — swept area of the rotor, π × (diameter ÷ 2)².
• v — wind speed in m/s at hub height.

This is the power carried by the wind through the rotor area, not what the turbine delivers. Convert mph or km/h with the mph to km/h converter and the km/h to m/s converter.

No turbine can extract all of the wind's power: the air must keep moving past the rotor. The theoretical maximum, the Betz limit, is 16/27 ≈ 59.3%. In the example above, at most ≈ 321 W of the 541 W at 5 m/s could be extracted in theory. Real turbines capture less, and the generator, controller and inverter lose more. That is why output must come from the turbine's measured power curve — delivered power at each wind speed — not from the formula.

A turbine's rated power is its output at a specific rated wind speed, which differs between manufacturers. Two "1.5 kW" turbines can produce very different energy at the same site if one reaches its rating at a lower wind speed. Compare turbines on their power curves and on estimated annual energy at a stated mean wind speed, preferably from an independent certification test, rather than on rated kW.

Instantaneous power (W) is not annual energy (kWh). Annual energy depends on how many hours the wind blows at each speed — the site's wind speed distribution — combined with the power curve.

Capacity factor = Annual energy ÷ (Rated power × 8,760 h)

Illustrative: a 1.5 kW turbine with an assumed 10% capacity factor produces 1.5 × 8,760 × 0.10 ≈ 1,314 kWh a year. The 10% is an input chosen to show the arithmetic; your site's value can be much higher or lower.

Because power follows v³, an annual mean wind speed alone understates or misstates output: a gusty site and a steady site with the same mean deliver different energy. Use measured or modelled hourly wind data where possible.

• Tower height — wind speed generally increases with height above the ground, and obstacles matter less higher up.
• Obstacles — buildings, trees and hills slow the wind and create turbulence downwind.
• Turbulence — gusty, changing wind lowers energy capture and increases wear.
• Exposure — open, exposed sites usually have stronger, steadier wind than sheltered ones.

Rooftop and urban sites are the hardest case. A UK field trial of small turbines (Energy Saving Trust, *Location, location, location*, 2009) found that building-mounted turbines in urban and suburban locations generally produced far less than at well-exposed free-standing sites. Treat any output figure for a sheltered or rooftop site with caution, and measure wind on site before deciding.

1. Wind resource at hub height — on-site measurement for a period, or a modelled dataset for the location.
2. Turbine power curve — preferably independently tested.
3. Annual energy estimate — distribution × power curve, minus losses.
4. Your use of that energy — compare with how much electricity you use and when; the DIY home energy audit guide helps.
5. Value — kWh used × import price plus kWh exported × export payment, from your own tariff.
6. Local rules — planning, permits and grid connection requirements depend on where you live; check with your local authority and network operator.

If you are also considering solar, the solar output guide uses the same annual-energy approach. The Home Energy hub collects related tools; sources are on the methodology page.