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Anemometer

You know those spinney things you sometimes see  on top of weather vanes?  They're called anemometers and they are measuring wind speed.  Wind speed is by far the most important factor in determining if a small wind turbine will work on your property.  Just because you see flags billowing every now and then doesn't mean you have enough wind to output a fair amount of power.  As I covered in the short slide-show on Wind Power Analysis, the most important formula to know when considering a wind turbine is the theoretical power in the area swept by the wind turbine rotor (the blades):

P = .5 • ρ • A • V³

• P = Power in Watts
• ρ (rho) = Air density = 1.225 kg/m³ at sea level
• A = rotor area exposed to wind in m³
• V = wind velocity in meters/sec²

But don't worry about knowing the equation in the preliminary stages of deciding whether you should buy a wind turbine.  The only thing you need to know at this point is "V" or the wind speed on your property.  And not just the instantaneous wind speed, but a range of values over the course of a few months.

You see, wind speed is so important because it is cubed in the equation above.  If you double your wind speed you don't double your power output, you increase it by a cubic factor! For instance, increasing wind speed from 4 mph to 8 mph is not double the power output, but increases it 8 times (8³/4³=512/64=8)!

While researching data from local airports or looking at national wind maps will be helpful, it really wont give you an idea of what the wind speed is at your house.  For instance, maybe you have trees in the way, or your house blocks the wind.  The only way to get a good idea of how much power a wind turbine on your property can produce is to get an Anemometer and preferably pair it with a data logger.  An anemometer will give you an instantaneous measure of wind speed and the data logger will take those instantaneous values and display them over time.

An example of a data logging Anemometer can be seen here from inspeed.  While a couple hundred dollar investment just to see if a wind turbine makes sense might sound like a lot, it is a much better investment than a multi-thousand dollar investment for a wind turbine that doesn't turn!  If your wind turbine installer is a good one, they might even have a data-logging anemometer you can rent.

Whether you use a data logging anemometer or not, make sure you have a good idea of the wind speed in the immediate vicinity of where you are planning on installing a wind turbine!