Selecting an Energizer for Your Electric Fence

Key points:

  • The most economical power source (hydro, batteries, or solar) depends on ease of access to fencer
  • Output voltage needs to be high enough to get the attention of the livestock being contained
  • Output joules rating must be capable of delivering the voltage along the length of wire in the fence
  • When in doubt, buy bigger than you need

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. Understanding electricity and all the jargon that goes with it can be a challenge. Luckily, thinking about water is often a great way to imagine electricity, so I’m going to draw comparisons between electricity and water throughout this article.

The first step in selecting an energizer is to determine your power source. If the fence is close enough, an energizer that plugs into the hydro is often the cheapest and easiest solution. As the fence gets further 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 that batteries are easy to change before they run down. For electric fences in more remote locations, a solar panel may be the best solution. These 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 4-6 hours of direct sunlight every day.

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. See Table 1 for minimum output voltages. Check that the energizer can deliver an appropriate amount of voltage.

Table 1: Minimum output voltages for different livestock and predator species

Species Minimum Voltage Comments
Cattle 2000 – 4000 Dairy cows are often very quiet and the low end voltage may be sufficient. Bulls can be aggressive and need higher voltage to contain them.
Sheep 4000 – 5000 Wool is an excellent insulator, so sheep fences need high voltage to ensure the shock is felt.
Goats 4000 – 5000 Some breeds of goats are well insulated – see comments on sheep
Horses 2000 – 3000 Stallions can be aggressive and need higher voltage to contain them.
Deer and Elk 3000 – 5000
Wolves and Coyotes 4000 – 5000 Some sources suggest a minimum voltage of 5000 for deterring predators.
Bears 5000

Compiled from Stafix and Rutland Electric Fencing

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. This is called loading. Your energizer must deliver enough joules to overcome the load and provide enough voltage along the length of the fence.

At this point, I need to remind you that fences are linear. Not because I think you don’t know that, but because some energizer manufacturers have forgotten. Many energizers state on the box that they will power a certain number of acres of fence. This is not helpful, because the length of a fence around a square 10 ac paddock will be less than one around a long, skinny 10 ac paddock (Figure 1). Measuring an energizer’s size in acres gets 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.

perimeter fence length changes with paddock shape
Figure 1. Perimeter fence length changes with paddock shape

The other point to bear in mind is that 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 actually 25 miles of wire (Figure 2). Many manufacturers claim their energizers can power 8-20 miles/joule of output, but these are numbers obtained under ideal laboratory conditions. In practice, if you have a one- or two- strand fence that is free from weeds, tall grass, and branches touching it, you might get 3-6 miles/joule. Under heavy loads (i.e. lots of debris touching your fence), you might only get 1 mile/joule. If you’re powering a multi-strand sheep fence, you might want a ratio of 0.16-0.33 miles/joule (between 3 and 6 joules per mile) to accommodate the high number of wires and the insulation value of wool.

energizers power miles of wire, not miles of fence
Figure 2. Energizers power miles of wire, not miles of fence

There are two different energy ratings you might see on the box. Output joules indicate the amount of energy sent through the fence. 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.

What’s a capacitor? Remember your power source? If the energizer is plugged in to an outlet, it’s receiving 120 V of electric potential (that’s your water pressure). If batteries or a solar panel is providing the power, likely the input voltage will be a multiple of 12 V. However, the output voltage is much higher – probably over 3000 V. The energizer contains a transformer, which creates that increase in electric potential. Capacitors store the higher electric potential and release it in pulses to the fence. Think of a tippy bucket at a water park. A small amount of water flows into the bucket, but once it gets full the bucket dumps the water out, rights itself, and starts collecting again. Capacitors build up electric potential and “dump” higher voltage down your fence line. However, not all the water in a tippy bucket gets dumped; there’s a little residual in the bottom when the bucket starts to refill. Capacitors have a similar residual, which is why stored joules are about 30% more than 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 onto your electric fence in future (either by fencing other fields, or subdividing existing paddocks), factor in how many more output joules you will require, and buy an energizer that can handle that future load. If you are in the process of expanding your fences, upgrading the energizer as you go gets expensive quickly.


This article first appeared in the Spring 2018 edition of Think Green!, the Ontario Forage Council’s newsletter