What you posted was voltage. You can get that at any speed from any quantity of magnets, the whole thing need only cost peanuts to generate volts.

What happens when you load it is a very different issue. When you produce a load curve of charge into a battery against speed and input power we shall have a good idea if your commercial built alternator is going to produce a good turbine.

The problems of low output that nmwindrider is having are typical of car alternator conversions and I am afraid that your alternator is not going to come out so well when you load it. You may be less tickled when you try to get real power out of it.

These little things are fine for small props and that means high winds to produce reasonable power. If you rate the thing in a 45mph wind it may be quite impressive but on the average site on the average day it may do very little.

There is no fundamental reason why you can't buy commercial things as cheaply as producing your own, they have the advantage of quantity purchase. What you pay for self build largely is determined by what you get cheap. If you go out to buy everything You may find the commercial price lower. If the end result is not satisfactory then you may not be better off.

There are a few reasonable quality Chinese import PMAs at very reasonable price and if I was going to buy something I would go that route rather than something based on a car alternator, but we all have our opinions and make our choices. I look forward to your experience with this thing so stick around and let us know the final verdict.