If you will be putting the wire in conduit, just go to Home Depot or Lowe's or a hardware store and get THWN-2 cable. The -2 is important, because it gives a higher temperature rating, which you'll need if the conduit is on the roof and/or being exposed to sun. The "W" is important because conduit is considered a wet location, water could collect in it. You'll need to look at the National Electrical Code (NEC) at your local library to learn about derating factors. For example, if all you can find is THWN (no -2) insulation, you may have to go with #8 AWG.
Option 2 is to go with USE-2, ultraviolet-resistant wire. In that case, you can go with #10 if it will handle the amps, and have it just exposed to the elements. I believe such an installation is code compliant, but it all comes down to the local inspector. If their experience is that all the pro's use conduit, you may have to have the NEC with you to defend your choice.
If this is for off-grid (no inspection) or just something temporary and fun, just use any wire from Home Depot. In my area, there is an electronic flea market that is cheaper, but that's unusual.