10 or 12! You will be fine running 18 gauge. Larger gauge won't hurt just more expensive and tougher to work with and conceal. Heck if you got some speaker wiring laying around that'll work fine. You don't need much for what your doing.
You gotta pay to play -Chip
12 is only 2mm is diameter, 16 is 1.2mm - it's not a huge difference and the larger gauge is easier to work with and crimp on connectors have more meat to bite, less apt to loosen when tugged, etc.... so yes, 10/12 is what I use.
The other consideration is how easy will it be for a squirrel to chew through? For that, 12 or 14 will give you a more conductors. The wires on an old Moultrie solar panel I had looked like about 22 gauge (not caliber). Just a nibble from the squirrels was all she wrote for that wire.