Electric Fencing

I've got some determined leghorn pullets that I'm hoping to keep free range, but they are really getting good at flying now. I'm going to run an electric wire over the top of my fence so that they can't perch up there and plot their escape. Does anyone have experience with electric fencing and chickens?

I'm worried if I buy a normal kit it will be juiced up for horses and the like and I'll end up harming the hens. If there are recommendations for energizers that would be great too.