One of the things that befuddles customers when they’re looking to purchase an energizer is the number of different ways that manufacturers rate the power of their products. Some of these measurements are field size, distance, stored joules, or output joules.
And here’s your top tip for the day.

Only one of these measurements is actually useful and tells you anything at all about the power output of the energizer. For example, distance measurements are a bit silly in that they don’t take into account the resistance in the wire or the interaction of the wire with other conductive materials in the environment. You’ll often see the manufacturer refer to optimal conditions, but then fail to specify what optimal conditions actually are. Regardless, this is the most common method of measuring energizer output power, and thus the one you are most likely to encounter. Despite its limitation, field size measurements, in my opinion, are a bit dumb and even more so than the distance measurements. A 1 ha field could have a perimeter of 400 meters or 1000 meters, depending on the shape of the field, and no allowance has been made for the resistance in the wires or even the number of wires in the fence. Field size measurements should be avoided.

The last two remaining options, therefore, are stored joules or output joules. The difference between stored joules and output joules is that stored joules are the amount of energy that is stored in the capacitor of an electric fence energizer, whereas output joules are the amount of energy that is delivered by the energizer to the fence wire. Stored joules and output joules are not the same because the transformer that converts the stored energy into a high voltage pulse is not 100% energy efficient.

Therefore, some of the stored energy is lost during the transformation process. In most cases, the output joules are approximately 70% of the stored joules, depending on the efficiency of the transformer. Output joules are more important than stored joules for determining the performance and effectiveness of the electric fence because they represent the actual energy that is being transmitted through the fence wire. Therefore, it is advisable to compare the output dual ratings of different energizers rather than the stored dualed ratings.

Why does all this matter?

Well, let’s say I wanted to deceive you. I could manufacture an energizer with a small solar cell, a large battery, a big capacitor, and a small or inefficient transformer. I could then advertise that the energizer has a big stored joule capacity, but the energizer would be woefully underpowered.
So the super summary when choosing an energizer solely on the basis of power, it is the output joules that matter.

We’ll touch the other aspects later.