Selecting an electric fence energizer is similar to selecting a tractor, says Erwin Quinn with Gallagher Fencing. For example, the size of the equipment you will be pulling generally determines the size of the tractor you will need   and tractors are rated and selected on horsepower, not RPMs.

With an electric fence energizer, first determine what species you want to control and then determine the total acres on which you want to control that species. The energizer selected must have enough power to control the animal, or animals, you want to control on the acreage required.

Final energizer selection, says Mr. Quinn, should be based upon the energizer’s joule rating under a 500-ohm load, and not based upon miles of fence or volts. Is the mile rating one wire for 50 miles or five wires for 10 miles? And does the mile rating actually contain animals for 50 miles or simply deliver detectable voltage? Buying an energizer on voltage is very similar to buying a tractor based upon RPMs   it simply will not do what it is intended to do, he adds.