OMAFRA Report: Selecting an energizer for your electric fence

A weekly report prepared by the staff of the Ministry of Agriculture, Food and Rural Affairs (OMAFRA).  If you require further information, regarding this report, call the Elora Resource Centre at 519-846-0941.  Office hours: 8:30 a.m. to 4:30 p.m. 

For technical information, call the Agricultural Information Contact Centre at 1-877-424-1300 or visit the OMAFRA website:


The energizer (also called a fencer) is the cornerstone of any electric fence. Selecting the right one for your farm is crucial to making an electric fence work for you.

The first step in selecting an energizer is to determine your power source. If the fence is close enough, an energizer that plugs into hydro is often the cheapest and easiest solution. As the fence gets farther away from an outlet, running wires becomes expensive and other power options start to look more attractive. 

Deep-cycling 12-volt marine batteries are another potential power source for an energizer. These work best when you have at least two to swap out – one battery can charge while the other powers the energizer. 

An energizer being powered by batteries needs to be fairly accessible, so batteries are easy to change before running down. For electric fences in more remote locations, a solar panel may be best.

Panels tend to be the most expensive power source for a fence, but because the panel recharges the battery, it’s less likely to lose power than batteries alone. For solar panels to be effective, they need to receive a minimum of four to six hours of direct sunlight daily.

An electric fence is a psychological barrier: livestock learn not to touch the fence because the shock is unpleasant. Untrained animals or a non-electrified fence may enable livestock to go right through the fence, because it is not physically strong enough to stop them. 

To ensure the fence is “hot” enough to convince livestock to stay in the paddock, you need enough voltage. Volts are a measure of electric potential. If we draw a comparison with water, voltage is akin to water pressure.

Energizers are rated in joules, which is a unit of energy. Your energizer must put out enough energy to deliver the right voltage along the entire fence.

Imagine a drip irrigation line. To be effective at watering plants, you need a high enough flow rate and water pressure to meet the water needs of whatever is at the far end of the line. The more holes you have in that drip tape, the less water will make it to the end. Those holes are comparable to weeds, tall grass, and branches touching your electric fence. They drain energy from the fence, reducing the voltage it delivers. That is called loading. Your energizer must deliver enough joules to overcome loading and provide enough voltage along the entirety of the fence.

Planning for power

Keep in mind: fences are not linear. Many energizers state on the box that they will power a certain number of acres of fence, which is unhelpful because the length of a fence around a square 10-acre paddock will be less than one around a long, skinny 10-acre paddock. 

Measuring an energizer’s size in acres is even less useful when you think about the different lengths of cross fencing it might have to power. So, it’s probably best to ignore acreage numbers on an energizer.

The other point to consider: energizers power miles of wire, not of fence. 

If you have a single strand fence that goes five miles, it requires much less power than a five-strand fence going five miles, because the multi-strand is truly 25 miles of wire. Many manufacturers claim their energizers can power upwards of 20 miles per joule of output, but these are numbers obtained under ideal laboratory conditions. 

In practice, if you have a one- or two-strand fence free from weeds, tall grass, and branches, you might get between three and six miles per joule. Under heavy loading, you may only get a single mile per joule. 

If you’re powering a multi-strand sheep fence, you might want a ratio of 0.16-0.33 miles/joule (between three and six joules per mile) to accommodate the high number of wires and the insulation value of wool.

There are two different energy ratings you might see on the box.

Output joules indicate the amount of energy sent through the fence, whereas stored joules are the amount of energy in an energizer’s capacitors – multiply stored joules by 0.7 to get an estimate of the output.

When in doubt, buy a bigger energizer. This is a point where it pays to think ahead. If there is a chance you will add to your electric fence in the future, factor in how many more output joules you will require and buy an energizer that can handle future load. 

If you are in the process of expanding your fences, upgrading the energizer as you go becomes expensive.