397 W/m² at 50 m: NREL wind power class 3 (“Fair”). Usable with modern low-specific-power rotors on tall towers. Using the mean speed alone (½ρv̄³) would underestimate it by 48%.
Wind power density at 50 m
397W/m²
3,476 kWh/m² of rotor area per year
NREL wind power class
3 · Fair
397 W/m² equiv. at 50 m
Air density
1.212kg/m³
-1.1% vs sea-level standard
Energy pattern factor
1.91
mean(v³) ÷ mean(v)³
Naive ½ρv̄³ (wrong)
208W/m²
ignores the speed distribution
Weibull scale c
7.9m/s
Most frequent speed
5.59m/s
Speed carrying most energy
11.17m/s
size rated speed around 1.2–1.5× this
Energy from winds ≥ 12 m/s
46.4%
mostly clipped by turbines at rated power
Equivalent at 10 m
199W/m²
IEC turbine class at 100 m: II
NREL wind power classes (Elliott et al., 1986)
| Class | Rating | W/m² at 10 m | m/s at 10 m | W/m² at 50 m | m/s at 50 m |
|---|---|---|---|---|---|
| 1 | Poor | 0–100 | 0–4.4 | 0–200 | 0–5.6 |
| 2 | Marginal | 100–150 | 4.4–5.1 | 200–300 | 5.6–6.4 |
| 3 ◀ you | Fair | 150–200 | 5.1–5.6 | 300–400 | 6.4–7 |
| 4 | Good | 200–250 | 5.6–6 | 400–500 | 7–7.5 |
| 5 | Excellent | 250–300 | 6–6.4 | 500–600 | 7.5–8 |
| 6 | Outstanding | 300–400 | 6.4–7 | 600–800 | 8–8.8 |
| 7 | Superb | 400+ | 7+ | 800+ | 8.8+ |
- NREL class speeds assume sea-level air density and a Rayleigh distribution (k = 2); the class is assigned here from power density, which also accounts for your k and air density.
- Wind power density is the power in the wind, not what a turbine extracts. Real rotors capture at most 59.3% (Betz limit), in practice 40–50% at the best speeds.