Part of DIY Wind Turbine
How to measure wind speed and energy potential at your site before investing weeks of labor building a turbine.
Wind Measurement
Why Wind Measurement Matters
Building a wind turbine without measuring wind is like digging a well without checking for water. You might get lucky, or you might waste enormous effort for nothing. Wind energy follows the cube law — if you double the average wind speed, you get eight times the energy. This means the difference between a site with 3 m/s average wind and one with 5 m/s average wind is not a 67% improvement — it is a 460% improvement. Small measurement errors translate into massive planning errors.
A minimum of one month of measurement, ideally a full season, gives you the data to make an informed decision. Should you build at all? How big should the turbine be? Where exactly should you place it? How much battery storage do you need? Without measurement, every one of these decisions is a guess.
The Beaufort Scale: Estimating Without Instruments
Before building any instruments, learn to estimate wind speed by observation. The Beaufort scale, developed in 1805, correlates wind speed with visible effects on land and water. It is surprisingly accurate once you calibrate your eye.
| Beaufort Number | Description | Speed (m/s) | Speed (km/h) | Observable Effects |
|---|---|---|---|---|
| 0 | Calm | 0-0.2 | 0-1 | Smoke rises vertically; no leaf movement |
| 1 | Light air | 0.3-1.5 | 1-5 | Smoke drifts; wind vane unresponsive |
| 2 | Light breeze | 1.6-3.3 | 6-12 | Leaves rustle; wind felt on face; vanes move |
| 3 | Gentle breeze | 3.4-5.4 | 12-19 | Leaves and small twigs in constant motion; light flags extend |
| 4 | Moderate breeze | 5.5-7.9 | 20-28 | Small branches move; dust and loose paper lifted |
| 5 | Fresh breeze | 8.0-10.7 | 29-38 | Small trees sway; crested wavelets on ponds |
| 6 | Strong breeze | 10.8-13.8 | 39-49 | Large branches move; umbrellas difficult; wires whistle |
| 7 | Near gale | 13.9-17.1 | 50-61 | Whole trees sway; walking against wind is difficult |
Your Target Range
A practical DIY wind turbine starts producing useful power at Beaufort 3 (gentle breeze, ~4 m/s) and reaches rated output around Beaufort 5 (fresh breeze, ~9 m/s). If your site rarely reaches Beaufort 3, wind power is not viable there. If it frequently hits Beaufort 5-6, you have an excellent site.
Building a DIY Anemometer
A simple cup anemometer can be built from salvaged materials in an afternoon. It will not be laboratory-precise, but it will tell you roughly how fast the wind blows at your site.
Materials Needed
- 4 small cups — plastic cups, cut-open soda cans, or halved ping-pong balls. They must be identical in size and shape.
- 2 rigid arms — wooden dowels, straight sticks, or metal rods, each about 30cm long
- A vertical shaft — a straight nail, bolt, or metal rod
- A base bearing — the shaft must spin freely. A drilled block of wood with the nail loosely inserted works. A salvaged ball bearing from a bicycle or skateboard is much better.
- A means of counting rotations — this is the key challenge
Assembly
- Attach one cup to each end of both arms, all facing the same rotational direction (all clockwise or all counterclockwise when viewed from above)
- Cross the two arms at right angles and fix them to the top of the vertical shaft
- Mount the shaft in the bearing so it spins freely with minimal friction
- Mark one cup with paint or tape to act as a reference for counting
Calibration
The relationship between rotation speed and wind speed depends on your cup size and arm length. Without a reference anemometer, use this approximation:
Wind speed (m/s) = circumference (m) x RPM / 60 x correction factor
The circumference is the circular path traced by the cup centers (2 x pi x arm length). The correction factor for simple cup anemometers is typically 0.3 to 0.4 — the cups spin slower than the wind. Start with 0.33 and cross-check against Beaufort observations.
Bicycle Speedometer Method
The most elegant DIY approach: mount a small magnet on one arm and a salvaged bicycle speedometer sensor near the shaft. Set the speedometer’s wheel circumference to the cup path circumference multiplied by the correction factor (0.33). The speedometer will then display wind speed directly in km/h. This gives you continuous, readable measurements without manual counting.
Mounting
Mount the anemometer at the height your turbine rotor will be — typically 10 meters. If you cannot mount it that high, mount it as high as practical and understand that wind at the actual turbine height will be faster. Wind speed increases with height roughly according to the power law: speed at height 2 = speed at height 1 x (height 2 / height 1)^0.14 for open terrain.
Recording Wind Data
Measurement without recording is useless. You need a systematic log to calculate averages and understand patterns.
What to Record
| Data Point | How to Measure | Why It Matters |
|---|---|---|
| Date and time | Clock or sundial, every 3-6 hours minimum | Reveals daily and seasonal patterns |
| Wind speed | Anemometer reading or Beaufort estimate | Core energy calculation input |
| Wind direction | Compass or wind vane | Determines turbine orientation and site layout |
| Duration of observation | How long you watched at each check | Weights your averages correctly |
| Weather conditions | Clear, cloudy, rain, storm | Correlates wind with weather systems |
| Notes | Gusts, calm spells, unusual events | Captures what numbers miss |
A Simple Log Format
Create a table in a notebook:
Date | Time | Beaufort | Direction | Duration | Notes
--------|-------|----------|-----------|----------|------
Mar 14 | 0700 | 2 | WSW | 5 min | Light, steady
Mar 14 | 1200 | 4 | W | 5 min | Gusty, dust lifting
Mar 14 | 1800 | 3 | W | 5 min | Steady, moderate
Mar 14 | 2200 | 1 | Variable | 5 min | Nearly calm
Do Not Cherry-Pick Measurements
It is human nature to check the wind when you notice it blowing and skip checks during calm periods. This biases your data high. Set fixed measurement times and check regardless of conditions. The calm readings are just as important as the windy ones.
How Long to Measure
| Measurement Period | Reliability | Suitable For |
|---|---|---|
| 1 week | Very rough estimate only | Deciding whether to keep measuring |
| 1 month | Gives seasonal snapshot, misses variation | Quick site comparison between candidates |
| 3 months (one season) | Good picture of one season | Reasonable basis for building decisions |
| 6 months | Captures seasonal transition | Good confidence in annual estimates |
| 12 months (full year) | Most reliable short-term dataset | Definitive site assessment |
For post-collapse practical purposes, one month minimum, one full season preferred. If you are deciding between two candidate sites, measure both simultaneously for one month — relative comparison is valid even with short datasets.
Calculating Average Wind Speed
Add up all your wind speed readings and divide by the number of readings. If you recorded Beaufort numbers, convert each to the midpoint speed first.
Example: Over 30 days, you took 120 readings (4 per day). The Beaufort numbers convert to midpoint speeds, and the average comes out to 4.2 m/s. This is your mean wind speed — a usable site for a small turbine.
The Average Is Misleading
Wind speed is almost never steady at the average. A site with a 5 m/s average might experience 0 m/s for 12 hours and 10 m/s for 12 hours. Because of the cube law, those 12 hours at 10 m/s produce 8 times the energy of 12 hours at 5 m/s. The actual energy available is higher than what the simple average suggests — but the calm periods mean you need battery storage to bridge the gaps.
The Cube Law: Why Speed Matters So Much
The power available in wind is proportional to the cube of its speed:
Power = 0.5 x air density x swept area x wind speed^3
In practical terms:
| Wind Speed (m/s) | Relative Power | Practical Meaning |
|---|---|---|
| 2 | 1x (baseline) | Almost nothing — below most turbine cut-in speeds |
| 3 | 3.4x | Minimum useful generation begins |
| 4 | 8x | Light charging, LED lighting possible |
| 5 | 15.6x | Moderate charging, small tools occasionally |
| 6 | 27x | Solid generation, power tools viable |
| 8 | 64x | Excellent output, most needs covered |
| 10 | 125x | Maximum practical harvest for DIY turbines |
This table reveals the fundamental truth of wind energy: a site with 6 m/s average wind produces roughly 8 times more energy than a site with 3 m/s average wind. This is why site assessment and measurement matter more than turbine design.
Estimating Monthly Energy Output
Once you know your average wind speed, you can estimate how much energy your turbine will produce. A well-designed small turbine captures about 25-35% of the theoretical wind power (this is called the capacity factor, limited by the Betz limit of 59.3% and real-world inefficiencies).
For a turbine with a 2-meter diameter rotor (common DIY size) in a 5 m/s average wind:
- Theoretical power: 0.5 x 1.225 x 3.14 x 5^3 = ~240 watts
- Realistic capture (30%): ~72 watts average
- Monthly energy: 72W x 720 hours = ~52 kWh
That is enough to run LED lighting, charge batteries, and operate small tools for several hours daily. Not luxury, but transformative in a survival context.
Wind Speed Distribution
Wind does not blow at the average speed. It varies continuously. Understanding the distribution tells you what your turbine actually experiences.
Most sites follow a Weibull distribution — a mathematical curve that describes the probability of each wind speed occurring. The key practical insight: the most common wind speed is below the average, and most of the energy comes from the relatively rare high-wind hours.
For a site with a 5 m/s average:
- Wind is below 3 m/s (too low for generation) about 30% of the time
- Wind is between 3-7 m/s (productive generation) about 50% of the time
- Wind is above 7 m/s (maximum output) about 20% of the time
- But that 20% of high-wind hours produces over 50% of total energy
Design for the Productive Middle
Optimize your turbine for the wind speeds that occur most often in the productive range (3-7 m/s for most sites), not for the rare peak gusts. A turbine designed for 15 m/s wind will barely turn in the 4 m/s breeze that blows most of the time.
Common Mistakes
| Mistake | Cause | Fix |
|---|---|---|
| Measuring only on windy days | Confirmation bias — checking when you notice wind | Set fixed measurement times regardless of conditions |
| Measuring at ground level | Convenience — anemometer mounted on a fence post | Mount at turbine hub height or apply height correction formula |
| Measuring for only a few days | Impatience to start building | Commit to minimum 1 month; 3 months is much better |
| Ignoring calm periods in calculations | Averaging only the windy readings | Include all readings, including zeros and calm periods |
| Assuming average speed equals constant speed | Not understanding variability | Calculate energy using the cube of each reading, not the cube of the average |
| Not recording direction | Only caring about speed | Direction determines turbine orientation and reveals site-specific patterns |
Key Takeaways
- Measure wind for at least one month before committing to building a turbine — one season is better
- The Beaufort scale lets you estimate wind speed by observation with no instruments at all
- A DIY cup anemometer with a bicycle speedometer provides continuous, readable wind speed data
- Record speed, direction, time, and conditions at fixed intervals — do not skip calm periods
- The cube law means doubling wind speed yields 8x the power — small speed differences between sites are huge
- A 2-meter DIY turbine in 5 m/s average wind produces roughly 50 kWh per month — enough for basic needs
- Most energy comes from the high-wind hours, but design your turbine for the common moderate speeds