I have done some work on my VAWT / Savonious Wind Turbine Calculator. It allows you to enter parameters and compute power output, amperage to batteries or the grid, cut in speed, etc. The program can be found here.
You start by selecting a battery voltage; 12, 24, or 48 volts. Alternatively, you may select 120 or 240 volts for grid tie. Weather conditions default to "moderate", but you may calculate for summer or winter as well. This is important because the temperature and density of the air can have a profound effect on the generator's cooling and therefore its maximum power output.
You can then enter the overall blade width. This is the width of the turbine as seen by its profile, not just one blade. Then enter the blade height.
The Gear Ratio field is used when your generator needs more speed to produce a usable voltage. Wind speed is entered in miles per hour. The calculations are based on this wind speed.
Tip Speed Ratio (TSR) is how fast the tips of the blades move in comparison to the wind speed. A typical horizontal axis wind turbine (HAWT) is usually between 3 and 10 TSR. A typical VAWT such as a Savonious is about 0.8.
The Blade Efficiency field also represents the Power Coefficient. A well-designed 2-blade Savonious has about a 24% Power Coefficient. A 3-blade has about 15%, although it does have more torque and works better at less than 23 mph or so.
The motor/generator's amp, volts, and RPM rating are usually given by the manufacturer. The generator's efficiency is a little different. Typically, a permanent magnet DC motor with brushes is about 50%. A three-phase permanent magnet alternator is about 75%, and an AC induction motor is about 90% (used for grid tie).