First RE System is Planned

Hi Eric,
In checking the NEC 2008 book, Table 8 Conductor Properties. Your suggested #2 wire in copper is .201 ohms/kFT and in Aluminum is .313 ohms/kFT.
The issue here is what is my wattage loss through this long cable run and what is the final voltage drop at the end of the cable run?  Ohms law and basic power formulas will give you these answers.
 
I squared * R = Power loss
and
E = I * R ,  for the voltage drop

Given: max amperage (I = 18.75 A)
    Max voltage (E = 48 vDC)
    Cable Ohms  #2 Copper@ .201 ohms per 1000 feet * 375 FT
                #2 Alu@ .313 ohms per 1000 FT * 375 FT
      Note: This being a closed loop DC circuit I must consider the cable run in both directions thus doubling my ohms total for the cable.
#2 copper ohms  (R = .15075 ohms total)
#2 aluminum ohms (R = .23475 ohms total)

Power Loss
I sq * R = 18.75sq * .15075 = 53 watts on copper
I sq * R = 18.75sq * .23475 = 82 watts on aluminum

Voltage Drop
E = I * R = 18.75 amps * .15075 ohms = 2.8 volts on Copper
E = I * R = 18.75 amps * .23475 ohms = 4.4 volts on Alu

Are these numbers tollerable??? Maybe!
Will your cable run be melting the snow between your wind tower and the garage with 53 watts? NO.
Will your wind mill be usually be outputting its maximum 900 watts and therefore 18.75 amps? No, so these numbers are based on a theoretical maximum which may not be achieved.
Will your batteries fail to fully charge? Yes, Your batteries really want 2.1v per cell ( 12.6 volts for a 6 cell battery). So hopefully the wind mill outputs just over 48 volts and the ocassions when your voltage drop over the wire is above 1 volt it will have little overall effect on your final delivered wattage to the garage.

Now increasing your cable size to 3/0 will give you a maximum voltage drop of 1.12 volts and only 21 watts of power loss.  Better? YES, but at what cost??? Price of #2 copper vs 3/0 copper.
MikeH.