It is largely a question of training the ewes. Cows are wimps when it comes to electric fences - one strand will keep killer bulls in. Pigs are easy too, intelligent animals with lots of bare skin, I have found problems moving them after the fence was taken down, they would not cross the line where the fence used to be! But sheep are trickier. They have a lot of fleece to insulate them from a shock, they panic easily, their one defence mechanism is flight, basically you get one shot at their nose and face until they learn what a fence is. I have a basic layout of permanent electric high tensile steel wire fences which I subdivide with 3 strand portable ones. I don't really like permanent fences - they create weeds. I also use paige wire which I believe you call pig wire or stock fence, this is ruinous to buy but I roll it up and reuse it in different locations throughout the year. I tie it to wooden posts with bale string. It is the best way with lambs until they are trained to electric barriers.
The key thing is the energiser. Mine cost me about $1000, it puts out 20 joules and as such is barely legal I think. Certainly a shock from it is something you only do once if possible as it is like being karate chopped on the back of the neck by a martial arts champion!
I have it insured against lightning strike which costs $8 a year. I have 6 earthing rods 20 feet from each other. Most people don't earth/ground their fencers adequately, you can almost double the output if you take the trouble. I have a solar powered energiser too which works well with older sheep in awkward locations.
The whole point is to get a good shot in so that the flock learn to keep off of the fence. I don't use electrified flexinet as I have had lambs tangled in it twitching as they get repeated pulses...Ugh!