Measure the voltage without a load and figure it will be a bit lower than that under load. If say you are running 36v unloaded, you will likely run 30v under load. This would be good for charging 2 12v batteries (to charge you need to run a higher voltage than the batteries). Then you can run a volt meter and an amp meter (Watts equals volts times amps). Most wind turbines are rated for a wattage at about 24MPH wind speed. As the wind speed decreases the output massively decreases so you might output 50w at 20MPH, but only 50w at 10MPH. That is the big drawback of wind power... you need wind and lots of it. The best way to do this is with a very tall tower or a VERY good location.
