Just showing another of the many ways to get this done, not saying this one is any better or worse, except the price seems good compared with some others around here (wire costs more here than in the States)
Home Depot here has this stuff at a low regular price. A guy at our campground last year got some for his small portable solar, so I checked it out.
You get four #12s in the one "wire" so my plan if I ever get any is to pair them into pos and neg pairs. Just twist the ends together-too easy. ISTR two 12s make a 9? Whatever. You buy however long you want off the reel.
Each wire is actually #12 stranded copper, not solid as shown here:
http://www.homedepot.ca/product/12-7-4-c-cu-twu-submersible-pump-cable-300m/908392
