I have a STAFIX SXS. Stafix SXS Solar Energizer by STAFIX | Southwest Agriculture Supplies
I have had it for a couple of years with no problems. The first year it was even in the shade for half the day(solar charged) and it still would knock you on your butt. Remember your ground rods are pretty important as with the amount of joules.
I bought this brand from the local co-op because of a recommendation from a friend who trades a lot of sale barn cattle who are soured and will not hesitate to bust through several strands of barbed wire. When he gets them home he "trains" them to the hot wire just ran around the perimeter of his pipe corrals. After a couple of days of testing his hotwire and learning he has no issue turning them out on the pasture. Of course his charger is an electric one and a much more expensive high powered model as cattle require a larger shock than horses, but I have not had a horse run through mine yet.( I think his puts out 2 joules? While mine is only .5 joules but gets the job done)