I'm honestly not sure why but it seems that most professional or heavy use systems are 24 volts. No idea why except perhaps the volts vs amps for wire size. 12 volts at 1400 amps theoretically gives the same power as 24 volts at 750 but you can use a smaller diameter wire on the latter vs the former.

I actually just picked up a book on solar electricity from a second hand store down here so I'm still trying to muddle through it. I try to find my answers to questions in the book but sometimes I have to take their word or do some online searching.

Till now I have only done simple house wiring under close supervision. However the guy I worked with did some contracting work in europe and apparently they run a much higher voltage at lower amps so they get the same power but can use thinner wire for everything. He said that the cost to wire a house was almost half of what it is over here because of less copper used.