The problem: nameplates that physics does not allow
Online marketplaces sell small turbines rated "3000 W" or "6000 W" with rotors half a metre wide. The power a rotor can extract is fixed by its swept area and by the wind, not by the label.
Declared
VAWT kit, rotor 0.53 m x 0.75 m (0.40 m²)
Physical limit at 10 m/s
Betz limit: 59.3% of the 243 W carried by the wind through that rotor
Realistic at 10 m/s
Typical power coefficient and drivetrain losses
How it works
Your anemometer
Point WhatIfWind at the wind sensor you already have in Home Assistant. It captures what grid weather data cannot: your height, your obstacles, your terrain.
Your turbine
Enter the rotor geometry from the datasheet, or the manufacturer's power curve. The nameplate is only used as a cap.
Real numbers
Live power, simulated energy, annual projection, capacity factor and a daily/monthly/yearly series, backfilled from InfluxDB if you have it.
A real case: two years on a rooftop
Anemometer on a roof at about 6.5 m, two years of hourly data (Nov 2024 to Oct 2026). Mean wind 0.74 m/s; only 2.6% of the hours averaged above 3 m/s.
| Turbine | Declared | Price | Simulated energy | Value per year |
|---|---|---|---|---|
| VAWT H-rotor 0.53 x 0.75 m | 6000 W | 329 EUR | 2.5 to 4.1 kWh/year | about 1 EUR |
| VAWT 0.53 x 0.75 m | 3000 W | n/a | 3.0 to 4.8 kWh/year | about 1 EUR |
Value at 0.25 EUR/kWh. At this site any turbine yields roughly 10 kWh per m² of rotor per year: even a large 10 m² rotor would earn a few tens of euros a year. The purchase was not made, and the money stayed in the pocket. Read the full case study.
Your site is a data point
Small wind is full of promises and short on measured results. If you run WhatIfWind, share what you found in Show and tell: a short form, no exact address needed.
- region and type of site, anemometer and its height;
- mean wind and the period analysed;
- turbines evaluated, simulated kWh per year, and what you decided;
- if you installed one: the real production against the prediction.