The optimum wind generator blade in terms of efficiency is a thin two bladed
prop. It will have about 70% efficiency. The power output depends upon
wind speed and will increase based upon the square of the wind speed.
However, a governor is required to prevent overspeed.

An efficient 68" two bladed prop would have about 5 watts output with a 5
mph wind, 100 watts with about a 14 mph wind and 500 watts with a 24 mph
wind.

I have both commercial and home built wind generators and what has been said
about using an aircraft propellor is true, they are not good wind generator
blades. It is fairly easy to carve a blade and I have made them with a
table saw and hand plane.

But, you cannot simply put the system up without a bit of thought. My first
wind generator I installed was a "temporary" one. Just a hand carved blade
attached to an electric motor. We normally do not have much wind, but that
night we had a 30 mph wind. The next morning I noted the blade was missing
on the wind generator. I looked for a month before I found the blade lodged
in a tree about 100 feet away.

Two bladed props are most efficient, but they create havoc. They shudder
when the wind changes direction and create stresses on the entire mechanism.
3 bladed props solve that problem, but are harder to make at home. Another
problem area is the gear boxes. Alternators like higher rpm than created by
the blades and the varying wind can put terrific loads on the gear boxes.
Many of the early commercial units suffered gearbox problems. A low rpm dc
motor can be used as an efficient generator. One of my home built designs
uses a 36 volt 900 rpm permanent magnet motor and works quite well.

Wind generators need a governor. Otherwise, high or medium winds will tear
it apart. In that situation, they can become lethal.

They are certainly fun to watch. Much more fun than a soap opera.

Colin