The WindWhisperer 100 Max cuts in at 2.2 m/s — a light breeze that most people can feel but barely notice. This is the wind speed at which the rotor begins to generate electricity. This is significantly lower than conventional small wind turbines, which typically require 3.0–4.0 m/s to start generating, and far lower than large utility-scale turbines.
But cut-in speed is only part of the story. What matters for your site's viability is mean annual wind speed — the average wind speed across an entire year. This page explains the difference, how to estimate your site's wind resource, and what it means for your installation.
Cut-in speed (2.2 m/s) is the minimum wind speed at which the rotor begins to generate. Below this speed, the aerodynamic forces are too small to overcome friction and start the rotor.
A low cut-in speed is valuable because it means the turbine generates energy during calm periods when other turbines are idle. The WindWhisperer 100 Max starts generating earlier in the morning, continues through light afternoon breezes, and captures energy during transitional weather that conventional turbines miss.
Mean annual wind speed is what determines whether your site is viable and how much energy you will actually generate. A site with a mean wind speed of 6.0 m/s generates roughly four times as much energy as one with 4.0 m/s. A site with 7.0 m/s generates roughly twice as much as one with 6.0 m/s.
Wind energy is proportional to the cube of wind speed — small differences in average wind speed create enormous differences in output. This is why the first step of any serious wind assessment is estimating your site's mean wind speed.
In the UK and Scandinavia, mean wind speed varies by location:
Sheltered lowland areas (towns, valleys, farmland without exposure): 4.5–5.0 m/s
Typical rural and semi-rural locations: 5.5–6.0 m/s
Exposed locations (coastal areas, hilltops, moorland): 6.5–8.0 m/s
Exposed coastal and upland sites (cliffs, exposed moorland, north-facing slopes): 7.5–9.0 m/s
The Department of Energy Security and Net Zero (DESNZ) in the UK publishes wind speed mapping by postcode. In Sweden, SMHI (Swedish Meteorological and Hydrological Institute) provides comparable data.
Below 4.5 m/s: A wind turbine is generally not economically viable. Other solutions — solar PV, heat pumps, building efficiency — typically offer better payback. The WindWhisperer 100 Max can still generate at these sites (it cuts in at 2.2 m/s), but annual output is too low for acceptable financial return.
4.5–5.5 m/s: Marginal. Payback period extends beyond 15 years at most UK and Scandinavian electricity prices. Worth considering only if electricity costs are exceptionally high or other factors (planning barriers to ground-level installation, existing roof capacity, solar integration) favour wind over alternatives.
5.5–6.5 m/s: Good-wind sites. Payback is typically 8–12 years. Most commercial wind installations fall in this range.
6.5+ m/s: Excellent-wind sites. Payback is typically 6–8 years. These are the most economically attractive locations — coastal areas, upland sites, exposed rural locations.
Wind speed increases with height above ground level. The exact relationship depends on local terrain roughness, but typically:
This is why raising a rooftop turbine by a few metres (taller rotor height) or installing on ground mast versus a short building matters. A WindWhisperer 100 Max at 4.4 m rotor height generates substantially more energy than one at 2.1 m.
For ground-level installations, mast height is a key decision. A 10-metre mast is easier and cheaper to install than a 15-metre mast, but a 15-metre installation reaches faster wind and produces 30–50 percent more energy. The optimal mast height depends on your site, surrounding obstructions, and economics.
Several approaches exist, ranging from rough screening to detailed assessment.
Online wind maps: The UK government's DESNZ wind speed mapping and Sweden's SMHI tool provide preliminary estimates by location. These are useful for a first check but assume average terrain and do not account for local obstructions.
Desktop assessment: A wind engineer reviews your building's location and height, nearby obstructions (other buildings, trees, hills), and local topography. This produces a refined estimate accounting for these factors and is more reliable than a map alone.
Anemometer data collection: For a final decision on a borderline site, many installers recommend installing a temporary anemometer (wind speed measuring device) for 3–6 months. It captures actual wind speed at your specific location and height and removes uncertainty. Anemometer rental typically costs £1,000–£2,000 for the equipment and data analysis.
For most commercial sites with reasonable exposure (not in a valley, not directly downwind of a tall building), a desktop assessment is sufficient. Anemometer measurement is most useful for marginal sites or sites with unusual local geometry.
Buildings, trees, and hills create wind shadow — an area downwind where wind speed is reduced by turbulence and friction. The size of the shadow depends on the obstacle height and the distance downwind.
A building 15 metres tall creates a wind shadow extending 200–300 metres downwind. A tree 10 metres tall creates one extending 100–150 metres. These rough figures show why location relative to obstructions matters:
Your building is upwind of obstructions: Good wind access. Wind approaches your site relatively undisturbed.
Your building is surrounded by similar-height buildings (town centre): Poor wind access. Obstructions in all directions reduce wind speed 20–40 percent below open-ground values.
Your building is downwind of taller buildings: Poor wind access. The taller building creates shadow that reduces wind speed.
Your building is on open rural land or coastal exposure: Excellent wind access. Minimal obstruction.
These factors are especially important for rooftop installations. A rooftop 20 metres high in an open rural setting has excellent wind access. The same rooftop height in a town centre surrounded by 20–30 metre office buildings has much poorer access. Your certified installer assesses this as part of the wind resource evaluation.
Every turbine has a power curve — a graph showing how much electricity it generates at each wind speed. For the WindWhisperer 100 Max:
The power curve shows that the turbine does not reach its full 100 kW rated output until wind speed reaches 10.5 m/s. Above that speed, it maintains 100 kW output (because the rotor is self-limiting by design). At lower wind speeds, output scales with the cube of wind speed — output at 6 m/s is roughly one-eighth of output at 10.5 m/s.
Annual energy output is calculated by summing the expected hours at each wind speed and multiplying by the power curve output. A site spending 200 hours per year at 6 m/s and 100 hours at 8 m/s produces far less energy than a site with identical hours distributed at 8 m/s and 10 m/s.
This is why sites with steadier, stronger wind (coastal areas, upland locations) produce far more energy than sheltered, gusty sites even at nominally similar mean wind speeds.
Ask a question to get a wind resource estimate for your location.
See also: How much energy will a vertical axis wind turbine produce? · Is my site suitable for a wind turbine?
Tell us your location and building height, and we will provide a preliminary wind resource estimate.
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