JC--,
A lot of questions-- i'll try my best to answer.
"Am i correct in assuming that 500 watts at 48v is the same as 2000 watts at 12v?"
No. 500W at 48V is the same as 500W at 12v. Watts is the total power produced. It's just that 48v at 500W is only 10.4 Amps (cooler;less line loss) instead of 500W at 12V is running at 41.6 Amps. (Total power in Watts= Volts x Amps)
I can understand only being able to go 20' high. (Esp. with that heavy motor)
My 5' mill is only 13' feet high because I HAVE to keep it low for now and it probably takes away Half the average watt Hours being this low. I hate doing it. But I suspect yours 20' location may not be as bad.
The blade diameter? I would be afraid of trying a 10' set at first.
I would try the 8' set and if it stalls early you can always add some resistance (power resisters or extra length of wire) to bring it out of stall.
But I am not that smart on this. If it happens, then simply tell it here, and those in the better know will tell you how much resistance to add.
It would be better and cheaper if you could make some (8' diam) pine blades first to see how the servo reacts in winds with this size prop.
And I highly suspect you can shut down (short the 3-phase wires together) an 8' set of blades with that servo in high winds when they come around, and the alternator will be able to KEEP it stopped no matter what giant winds hits it. I am not so certain of the bigger 10' set. (Make certain you have VERY good secure connectors and connections on the alternator)
They say 48V system is best anyway. I think That alternator will stall early if you try 12v or even 24v system. (The cut-in would be much Too low)
I believe Around 170-180 rpm for a 48v system is a decent cut-in for an 8' diam mill.
It would be good if you could verify by hand-cranking the servo at around one rotation per second (60rpm) to see if it puts out (after rectifiers) around 16 volts. If it don't, then you need to say something about it here. If it only puts out about 8v per rps, then it may be better suited for a 24v system.
You will need a 48v Charge Controller (Outback ;MorningSTar;etc) and a more expensive 48v power inverter.
Yes, the tail pivots upward (according to its weight and length) on an axis and as you say, causes the blades to turn out of the wind direction.
I have never even made that type of furling yet, but I hear it is not as simple as it sounds. It will probably take time to fine tune that.
"Are there inverters that let you draw a load straight from the turbine and use the excess energy to charge batteries?"
I'm not certain. If you are refering to 'Grid Tie' inverters, I do not know.
The common type of inverter has to have batteries in order to be regulated and work correctly.
-Don't feel too bad, the giant servo motor I got weighs 90lbs + 5 lbs for the steel arbor mount. Strained my back for a week lifting it up on an 8' testing pole with a step ladder by myself. Felt like I was lifting Opra up by her feet!! Ouch.
Hope you have friends and make a very sturdy tower.
-Hope you learn a lot--and don't ever get discouraged.