Don't be fooled by the Joules rating.
From Wikipedia:
"In terms firstly of base SI units and then in terms of other SI units, a joule is defined below, where kg is the kilogram, m is the metre, s is the second, N is the newton, Pa is the pascal, W is the watt, C is the coulomb, and V is the volt:
J = kg ⋅ m 2 s 2 = N ⋅ m = Pa ⋅ m 3 = W ⋅ s = C ⋅ V
One joule can also be defined as the following:
The work required to move an electric charge of one coulomb through an electrical potential difference of one volt, or one coulomb-volt (C⋅V). This relationship can be used to define the volt.
The work required to produce one watt of power for one second, or one watt-second (W⋅s) (compare kilowatt-hour – 3.6 megajoules). This relationship can be used to define the watt."
Look at the Joule. It has a time component. If you increase the time, you increase the Joules. So if your electric fencer is on for 40 microseconds instead of 20 microseconds, you just doubled your Joules. This does not make it a better fencer. I prefer the rating system of volts at 500 ohm load.
Look at Parmak's models.
https://parmakusa.com/product-category/complete-system/
https://www.jefferspet.com/search?utf8=✓&animal[]=&q=parmak&button=search