Yeah, it's safest to have it hot. Really mild horses might not test it, but most horses do at any given point. Even if it's just for a little while so they discover that it's hot. Then you could probably turn it off and they should still respect it. Otherwise, they'll discover that it won't shock them and they could get tangled in it. I guess it really depends on the horse. If you're not near an outlet, you could get a solar pannel for it.