Why Tower Height Decides Whether a Turbine Pays
Wind gets stronger with height
Near the ground, trees, buildings and the ground itself slow the wind. Higher up it runs faster and steadier, and a turbine's output rises with the cube of wind speed, so a small gain in speed is a large gain in power. That is the whole case against rooftop and short-pole turbines.
One typical county, three heights
Emanuel County, GA sits at the national middle for wind. There the same 10 kW turbine makes:
| Hub height | Average wind | Output | Payback |
|---|---|---|---|
| 10 m, about roof height | 3.58 m/s | 4,645 kWh | over 100 years |
| 30 m | 4.79 m/s | 11,163 kWh | 58 years |
| 50 m | 5.49 m/s | 15,965 kWh | 41 years |
Across the country
At 10 m, a turbine pays back inside its life in 13 of 3,144 counties. At 30 m that rises to 50. Going to 50 m would add another 76, though a 50 m tower costs much more than the model's and many counties cap tower height well below it, so check zoning before you price one. Among the counties a 50 m tower would turn around: St. Clair County, MI and Emmet County, MI.
What this means for rooftop kits
A roof is the worst place for a turbine: low, and sitting in the turbulence the house itself makes. The table above uses clean 10 m wind, which a rooftop does not get, so a real rooftop turbine does worse than the first row. If a seller is offering one as a way to cut your bill, ask them for measured output from a comparable installation. Height is only half of it: wind or solar shows where panels beat a turbine outright, and how the numbers are made gives the shear method.