A 5 m/s postcode average understates what a commercial rooftop can support — the 2.2 m/s cut-in and rooftop upwash effect produce more generation hours than the open-field figure suggests.

Property owners who check a wind resource map for their postcode and see a 4.5 to 5 m/s average wind speed often conclude a turbine is not viable. Three factors change this calculation — and none of them appear in the postcode average.

The cut-in speed difference

Conventional horizontal-axis turbines (HAWTs) typically require 3 to 4 m/s to begin generating, and their output ramps slowly from cut-in to rated speed. The WindWhisperer range begins generating at 2.2 m/s.

At a site with a 5 m/s average wind speed, the wind is below 4 m/s for a significant proportion of the year — hours during which a HAWT generates nothing and the WindWhisperer is already producing. The lower cut-in speed translates to more generating hours per year, not just a marginal difference on a few days. The annual output figures for the range already account for this: approximately 115 MWh per year for a 50 kW unit; approximately 230 MWh per year for a 100 kW Max unit, based on a representative northern European wind profile.

The rooftop upwash effect

Wind approaching a building is deflected upward as it reaches the roof edge, creating a local wind acceleration at rooftop level. This upwash effect means that the wind speed at the rooftop of a commercial building is typically higher than the open-field average for the same location — sometimes significantly higher, depending on building height, roof geometry, and the prevailing wind direction relative to the building orientation.

A postcode wind resource figure is measured at open-field height and does not capture this effect. A turbine sited at the leading edge of a commercial rooftop in the prevailing wind direction benefits from the upwash — producing more generation than a ground-level or open-field installation at the same location would suggest.

The specific upwash factor for a given building depends on its geometry and is assessed in HNordic's site-specific wind resource evaluation.

What the annual output figures already assume

The approximate annual output figures for the WindWhisperer range are not rated-output projections — they are not calculated by multiplying rated power by 8,760 hours. They represent the expected generation at a representative northern European wind profile, accounting for the full distribution of wind speeds across the year: the many hours at low wind speed, the moderate-wind hours that deliver the bulk of generation, and the infrequent storm periods.

A property owner at a site with a 5 m/s postcode average can compare their location's actual wind speed distribution against the representative profile. In most commercial locations in England, Scotland, Sweden, and other northern European markets, the representative profile is a reasonable approximation of what is available.

Key takeaways

• The 2.2 m/s cut-in speed produces more generating hours per year at low-wind sites than a conventional HAWT, which requires 3 to 4 m/s to begin generating.
• The rooftop upwash effect typically increases local wind speed above the open-field postcode average — a rooftop is often a better wind site than the postcode figure implies.
• Annual output figures for the WindWhisperer range already account for the full wind speed distribution — not just hours at rated speed.
• HNordic's site assessment models the specific wind resource at the property's rooftop, not the postcode average, and produces specific output figures for the system it recommends.

See also: A wind turbine with no cut-out wind speed — how does it protect itself? · Does the wind turbine require planning permission? · Full FAQ