Since electricity is not something that can be seen, it is difficult for many people to understand how it works. Unfortunately the industry has not developed a universal method of statistics to allow a consumer to make an educated decision on his purchase.

The following terms are often used to evaluate and compare energiser units.

• Volts: Volts are a unit of measurement used to describe an amount of electrical potential, or "pressure". The higher the voltage-the higher the pressure. In an electric fence system, a high voltage is important for making sure that an electrical charge can bridge tha natural resistance and find its way through the hair on the animals it is intended on either keeping in or out.
• Joules:A joule is a unit of electrical energy equalling the amount of energy required to produce one watt for one second. The equation is:watts x seconds = joules. The higher the volts and amps that an energiser can deliver, the higher the watts. And the higher the watts delivered in the same pulse time, the higher the joules. These equations are important to understand when comparing energisers.
• Miles of fence: The standard of "Miles of Fence" in describing the power of a fence controller has been in use for many years. Although many companies still use this standard in evaluating their energisers, a unit which is rated as a fifty mile unit only means that it can effectively power 50 miles of a weed free, properly insulated, single strand of 14 to 18 gauge wire that is 30 to 36 inches off the ground. It does not take into consideration the weed loading or the electromagnetic resistance that occurs in multiple wire fences. A 50 mile energiser will not necessarily power 10 miles of five wire high tensile fence. Although an energiser rated in "Miles of Fence" may be as good as a unit rated in joules, it is difficult to make a fair comparison using this method of rating. 

Follow this link for a more in depth review of comparing Electric Fencing Energisers.