If they are 12 volt nominal panels rated around 3 amps each, then you could look for a MPPT controller that can take in up to about 150 volts and put out up to about 25 or 30 amps at 12 VDC (that can be 13.5 volts normally).
In this way, you only have about 3 amps going through the wire, and practically no voltage loss across the wire due to amperage. Using #10 wire will lessen any voltage loss even further. The 6 panels would be in series, so that you connect the negative wire to the first - terminal on panel #1, and continue with only 1 wire from +12 of panel #1 to -2 then +2 to panel # 3, 4, 5, and 6. + output of #6 panel then returns to the controller. On a cold sunny day, it might see as many as 130 volts, perhaps as much as 150 volts. Under charge load, it will probably see about 100 volts X the amperage rating of your panels (say 2.5 volts) and at the controller this 100 volts input is changed to 13.2 - 13.5 volts to charge the battery bank.
I would recommend #10 UV rated direct burial wire from Home Depot. For mounts, I would suggest aluminum angle from Home Depot. On the roof of my motorhome, I cut 6" long and drilled three holes for #10 screws into the roof, and 5/16" hole for the bolt into the panel frame.
If I where to wire it up with a lower cost PWM controller, then I would run two wires from the panels to the controller, with 3 panels on each line. This would put about 9 amps on each run of #10 wire, you can go as high as 15 or 16 amps, but to put the whole 18 or 19 amps on #10 wire, you might see 5% or 8% loss due to high amperage on the wire.
If you find that you want to expand the system, this place sells panels for about $1 per rated watt.
SunELec.comGood luck,
Fred.
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