Wind power rises with the cube of wind speed, so a site’s average speed decides whether a turbine is worth building: below 3 m/s do not build, from 4 to 5 m/s a turbine is practical for battery charging, and above 7 m/s the site is excellent. Measure for at least 3 months, ideally a full year, at the height where the rotor will sit, because a 6 m/s site gives (6 ÷ 5)³ = 1.73 times the energy of a 5 m/s site.
Part of Advanced Wind Turbine
A wind resource assessment is the measuring and judging of how much wind a site has before a turbine is built on it.
Speed Counts Three Times Over
The power in moving air passing through a rotor is P = ½ × ρ × A × v³, where ρ is the air density (1.225 kg/m³ at sea level), A is the swept area of the rotor in m², and v is the wind speed in m/s. Speed is cubed and diameter is squared (A = π × r²). From this the Wind Power page draws the key fact: doubling wind speed gives 8 times the power.
Wind is a cube law; hydro is a straight line
A wind site that is 44% faster gives 1.44 × 1.44 × 1.44 = 2.99, about three times, the power. In hydro, power is proportional to flow and to head, so a 44% larger flow gives 44% more power (Power Calculation). A stream changes over weeks; wind changes hour by hour. Never use the head-times-flow rule for a rotor, and never use the cube for a stream.
The carrier grades sites by average wind speed:
| Average wind speed | Viability |
|---|---|
| below 3 m/s | not worth building |
| 3-4 m/s | small system, limited output |
| 4-5 m/s | practical for battery charging |
| 5-7 m/s | good: full household or small community power |
| above 7 m/s | excellent: significant community power |
Between a 5 m/s site and a 6 m/s site, the energy is in the ratio (6 ÷ 5)³ = 1.73, a gain of 73% for a 20% rise in speed. That is why a hilltop or a taller tower can matter more than a better blade (Site Selection & Tower Height).
Method: Measure Before You Build
Step 1: Observe for 2 to 4 weeks without instruments. The carrier uses Beaufort observations: leaves rustling is 2-3 m/s, small branches moving 4-5 m/s, large branches moving 6-8 m/s. Permanently bent trees mark a site with consistent strong wind.
Step 2: Build a cup anemometer. Four cups on horizontal arms on a vertical shaft; count rotations per minute and calibrate against a known wind speed (Wind Measurement uses a bicycle speedometer with a correction factor).
Step 3: Mount it at the height the rotor will sit. Wind near the ground is slower and rougher.
Step 4: Log for at least 3 months, and preferably a full year to cover all seasons. Hang streamers at tower height to see the pattern.
Step 5: Work from the average and the distribution, not only the average: Average Wind Speed & Power Curve shows why.
What The Result Means In Watts
The carrier gives a 2 kW turbine at a 5 m/s average site about 300-500 W average, a capacity factor of 15-25%. That is 0.3 × 24 = 7.2 to 0.5 × 24 = 12 kWh a day. For comparison, the carrier’s 5 L/s, 10 m hydro example makes 350 W all day, 8.4 kWh a day, with no calm periods. Every wind site has multi-day calms, so the carrier says a system needs 3-5 days of battery, a backup generator or a complementary source.
Check for water first
The carrier says that where a stream with 3 or more meters of head is available, micro-hydro is more reliable and cheaper per watt than wind (Micro-Hydro Turbine).
Common Mistakes
- Judging the site from ground-level wind — a rotor 20 m up sees faster, smoother air.
- A single week of data — wind changes with the weather and the season.
- Counting the average speed as the usable power — power depends on the cube of speed.
Safety
Mounting the anemometer
Putting an instrument on a mast or tree means working at height. Use a tilt-up mast that can be lowered (Tilt-Up Design for Maintenance) and never climb in wind or rain.
What’s Next
- Learn next: Average Wind Speed & Power Curve to convert speeds into energy
- Combine with: Blade Design & Aerodynamics once the site is proven