Why not use a normal battery?
That's not how an electric fence works. Connecting directly to mains or to a 12v battery source does not safely or effectively energize a fence.
The fence needs to be attached to an energizer, which takes a little bit of low voltage energy, and pushes it down the...
Mile ratings on an energizer is a curious manufacturer's rating. It applies to a single strand of conductor w/o any grass contact, excellent soil conditions (moist/conductive) and proper grounding. It does not apply to real-life fences or netting with its many conductors and potential for grass...