1Your site

Average annual wind speed

From a wind map, nearby airport or anemometer.

Measured at height (m)

Turbine hub / tower height (m)

Terrain around the tower

Site elevation (m)

2Turbine

Typical model class

Rotor diameter (m)

Rated power (kW)

Cut-in / cut-out speed (m/s)

Overall efficiency Cp (rotor + generator)

Small turbines: 0.25 – 0.35. Physical limit (Betz): 0.593.

3Your needs

Electricity use (kWh / month)

Electricity price (per kWh)

Installed cost (turbine + tower + wiring)

Incentives / tax credit (%)

Weibull shape factor k

2.0 = Rayleigh (typical inland). Coasts often 2.2 – 2.6.

Annual energy––

Capacity factor––

Wind at hub height––

Covers your use––

Hours per year at each wind speedTurbine power curve (kW)

How the calculator estimates wind turbine output

The power available in moving air rises with the cube of wind speed. Double the wind speed and the energy in the wind increases eight times. This single fact explains why site selection and tower height matter more than almost anything printed on a turbine's spec sheet.

P = ½ · ρ · A · v³ · Cp

Here ρ is air density (about 1.225 kg/m³ at sea level, lower at altitude), A is the swept area of the rotor (π × D² / 4), v is wind speed and Cp is the overall efficiency of the rotor, gearbox and generator. A 3.7 m rotor sweeps 10.8 m²; at 6 m/s with Cp = 0.32 it produces about 460 W, while at 12 m/s it reaches its 3 kW limit.

Why the average wind speed is not enough

Wind is never steady. A site with a 5 m/s average spends many hours below the cut-in speed and some hours in strong winds that carry a large share of the yearly energy. Because of the cube law, the average of v³ is roughly twice the cube of the average speed. Rotorwise therefore spreads your average across a Weibull distribution (shape factor k = 2 is the classic Rayleigh case), applies the turbine's power curve to each wind speed and sums the result for all 8,760 hours of a year.

Adjusting for height and terrain

Wind maps and weather stations usually report speed at 10 m. Wind gets stronger with height because the ground slows it down. We use the wind shear power law v_hub = v_ref × (h_hub / h_ref)^α, where α ranges from about 0.11 over open water to 0.30+ over forests and towns. Raising a tower from 10 m to 24 m over farmland typically increases wind speed by 19 % – and energy by roughly 60 %.

A practical rule used by installers: the bottom of the rotor should be at least 9 m (30 ft) above anything within 150 m (500 ft). Below that, turbulence from buildings and trees reduces output and shortens the turbine's life.

What do the results mean?

Wind at hub height	Site rating	Typical capacity factor	Verdict
below 4 m/s (9 mph)	Poor	under 8 %	Solar is almost always a better investment
4 – 5 m/s (9 – 11 mph)	Marginal	8 – 15 %	Only for off-grid hybrid systems
5 – 6 m/s (11 – 13 mph)	Fair	15 – 22 %	Can pay back with incentives and a tall tower
6 – 7 m/s (13 – 16 mph)	Good	22 – 30 %	Solid investment for rural properties
above 7 m/s (16 mph)	Excellent	30 % +	Wind clearly competitive with solar

Capacity factor is the energy actually produced divided by what the turbine would make running at full rated power all year. It is the best single number for comparing turbines and sites; manufacturers' "up to" claims ignore it.

Choosing a turbine – what to compare

Swept area, not rated power. Two "3 kW" turbines can differ by 50 % in annual output if one has a bigger rotor. Compare kWh per year at your wind speed.
Certified power curve. Look for certification to IEC 61400-2, SWCC/ICC-SWCC (North America) or MCS (UK). Uncertified curves are often optimistic.
Rated annual energy at 5 m/s – a standardized figure printed on certified labels.
Cut-in speed and over-speed control (furling or pitch) for storm survival.
Tower height and type – guyed lattice, tilt-up or monopole. The tower often costs as much as the turbine.
Grid-tie vs. battery charging. Off-grid turbines need a charge controller and dump load; grid-tie units need an approved inverter.
Sound level at 60 m and permitting rules for height and setbacks in your area.

Wind + solar hybrid systems

In many climates wind is strongest in winter and at night, exactly when solar panels produce least. A small turbine paired with a solar array and a battery can significantly reduce the battery size needed for an off-grid cabin. If the calculator shows a marginal site, compare the cost per kWh with a solar estimate before committing.

Frequently asked questions

How much electricity does a 1 kW wind turbine produce?

At an average hub-height wind speed of 5 m/s a typical 1 kW turbine with a 2.5 m rotor produces roughly 1,000 – 1,400 kWh per year. At 4 m/s the figure can fall below 700 kWh.

What wind speed is needed for a home wind turbine?

Most experts recommend an annual average of at least 5 m/s (11 mph) at hub height for a grid-connected turbine to make financial sense.

Can a roof-mounted turbine power my house?

Rarely. Wind over roofs is slow and turbulent. Field trials of building-mounted micro turbines in urban areas recorded capacity factors under 5 %. A free-standing tower clear of obstacles performs far better.

Are vertical-axis turbines better?

Vertical-axis designs accept wind from any direction and can be quieter, but they generally have lower efficiency (Cp) than horizontal-axis turbines of the same swept area. Use a lower Cp (0.20 – 0.25) when modelling them.

How accurate is this estimate?

With reliable wind data the method is typically within ±20 % of measured output. The biggest uncertainty is the wind speed itself – a year of on-site anemometer data is the gold standard.

About Rotorwise

Rotorwise is an independent, free tool that estimates the annual energy output of small and residential wind turbines from your site's wind speed, hub height, terrain and turbine specifications. We are not affiliated with any turbine manufacturer, installer or utility.

Every formula on this page runs entirely in your web browser. The numbers you type are never uploaded, stored or shared. Our goal is simple: give you an honest starting point so you can compare product specifications with confidence and avoid paying for capacity you do not need – or buying something too small to do the job.

The methodology combines widely published engineering rules of thumb, manufacturer data sheets and the electrical code practices that installers use every day. We review the assumptions regularly and update the calculator when typical product specifications change.

Contact

Found a bug, have a suggestion or want to report an outdated assumption? We read every message. Write to us at contact@electricity-and-accessories.shop and we will usually reply within two business days. Please do not send personal, financial or sensitive information by email.

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Last updated: October 28, 2026

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The calculator and articles on this website are provided free of charge for general information and educational purposes only. Results are estimates based on typical values and simplifying assumptions; real-world performance depends on the specific product, installation, climate, wiring and how you use your equipment.

Nothing on this site is professional engineering, electrical or financial advice. Always follow the manufacturer's instructions and local electrical codes, and have permanent installations, transfer switches and grid-connected systems designed and installed by a licensed electrician or qualified installer. Never operate fuel-burning generators indoors or in enclosed spaces – carbon monoxide can kill within minutes.

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