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Don's Home Home & Garden Electric Fence

Typically electric fences are used to contain livestock, but they can also be used to Protect Rabbits, Poultry, Dogs from Coyotes, Raccoons, Foxes and to keep Keep Pests out of the Garden, Flower Beds...

Electric fence chargers are rated by joules, miles and acres.
ranging from
A small garden one 0.25 joules 8 acres 2 miles 3k volts
larger farm cattle, horses, sheep ... 5-20 Joules 50 miles 100 acres
Higher Joules are for longer distances not necessarily a higher voltage.
Shocking voltage is usually 2k V - 7K V, with a pulse every second of 200 milli amps (thousands of an amp)

Energizers generally have a capacitor that serves as a storage tank for electrical energy. The energy builds up in the capacitor until it reaches a set level. At that stage, it is permitted to discharge into the transformer, which takes that energy and raises the voltage while reducing the amperage (often called amps). With reduced amperage, a safe electrical shock is transmitted through your fence. The higher the amperage, the more dangerous the shock.
Transformers are usually about 60-70% efficient, so the output joules is lower than the stored joules. Some manufactures list both and others don't say which one they are listing.

See What are electric fence volts, Amps and Joules.

* Joule - A "joule" is a unit of energy, like a "watt hour", but represents a much smaller unit of energy, with one watt hour equaling 3600 joules;
Essentially, a watt hour is the energy used by a 1 watt device over the course of one hour.

Low impedance chargers are the most common fence chargers manufactured today. They are much safer than their predecessors, the high impedance chargers that used an AC high voltage output.

A short high voltage low amperage short shock which will startle but not injure the animal or people.

The work with a grounding rod in the ground and wire strands or netting above ground.

Typical Low Impedance System:

Pulse Rate 1 per second with
           a duration of 0.0003 seconds
Voltage    3,000 - 5,000 V
Amps       100–150 milliamps

I have this one - Happy Henhouse Charger $32
0.25Joules 8 acres

They also have a 15 Joule one which would have been enough, but I couldn't find it online.