Regardless of power level, or being activated at all, a sign with a lightning bolt and a handful of zeroes are usually enough to deter intruders. This is the psychological effect of electric fence signs, not even the electric fence itself. For beings who are not people, however, these signs are moot and sometimes even food. Farmers around the world use electric fences to keep their livestock from straying, but this is the instance where power levels matter.
The rule of thumb for fence voltage is that, if it is over 9000, then you have gone too far. Contrary to popular belief, monkeys cannot handle that sort of power. The most they can handle is between 1000 and 2000 volts. The same goes for dairy cattle, while beef cattle, pigs and horses can take a thousand volts more as they have more muscle and fat.
Livestock with thick coats possess higher tolerance. Sheep, goats, deer and elk can take 4,000 to 5,000 volts of electricity. The most resistant animals are the wild ones, which farmers would want to keep out using a second exterior fence. Wild hogs are aggressive and persistent, while bears require highly visible fences to keep them from unknowingly running through them. Both will require at least 5,000 volts before retreating.
If you recoiled while reading the last two paragraphs, hold out calling PETA just yet. These numbers are the standard, closest voltage fence providers and professionals, such as Direct Trades Supply, recommend. They may seem too much for us humans since we can take as low as 110 volts for us to be seriously injured, but they are harmless to these creatures.
Animals lack the pre-emptive sense of preservation we have, so they require as much as what would register to the part of their brains which tells them to go back. This is part of why they can eat the things we could never even touch, and do feats we can only dream of achieving. Their physiology simply allows them to handle more electricity. A grizzly may not even feel a 4,000-volt fence, ending up entangled in the metal and in even greater danger.
Farmers would never profit if they did not care for their livestock, considering electric fencing is the practical way for animals to experience and graze on the open range. Electric fencing is not a way for people to establish dominance, it’s a benign compromise to keeping animals in warehouse cages for all their life. It is a way to tell forces beyond our control to back away, without us having to deliver the message ourselves. Because whether we admit it or not, we are quite afraid of these forces. They have power levels of over 9000.