What is a Joule in Electric Fence

As a beginner, I also didn’t know what a joule is in an electric fence. It’s the most basic concept behind the entire electric fence system. Joule is the energy or heat used to deliver current in the fence wires.

In this post, we’ll discuss what is a joule in an electric fence and how it impacts the performance of your electric fence.

Let’s understand some basics first:

What is a Joule

Joule is the SI unit of energy that is utilized(released) when one Newton force is applied to displace(move) an object for one meter. For example, you lift/move a ball weighing one newton to one meter, then 1 joule of energy is used.

In electrical terms, One joule is the energy released in a second when one-ampere current passes through a conductor having one-ohm resistance.

According to the joules law,heat(joules) produced in a circuit is directly proportional to the current, resistance, and time.”

  1. H (Heat or joule) = I (Current) x V (Voltage) x T (Time)
  2. H = I x I x R (Resistance) x T

A watt is the SI unit of power and is equal to the energy(joule) a device burns per second.

1 watt (power)= 1 joule(energy consumed) / second

For Example, if you light a hundred-watt bulb, it’ll consume 100 joules of energy per second.

What is a Joule in an Electric Fence?

Fence energizers are rated in joules, which is the unit of energy. Joules is the energy consumed or required to deliver/push(penetrate) voltage in the entire fence wires. The higher the joule rating is, the more powerful a fence energizer will be.

Let’s understand this with a general example of calories. If you carry 5 kg weight to 1 km, let’s suppose your 100 calories are burnt. If you increase the weight(10kg), you’ll need more calories to complete the task.

In the same way, if there is less load on the fence, fewer joules will be required. If there is excessive vegetation, dry ground, and poor conductors used, more joules will be required to deliver current in the fence wires.

Many beginners will ask how many joules should an electric fence have.

The recommended joules requirement is at least one joule per mile, but if you can deliver more joules, your fence will be more effective.

In ideal conditions, you’ll need very few joules(energy) to power the entire fence. However, factors such as vegetation, dry soil, length of wires, and poor conductors make it hard to maintain the desired charge in your fence. The more the external influence is, the more joules will be required to push voltage in the wires.

Fence chargers are designed for different purposes, but according to experts, under heavy loads(vegetation and long fences), a fence charger must output at least one joule per mile. The joules requirement increases as the duration of the voltage pulse, resistance, and length of fence wires increases. 

Fence chargers deliver current/voltage in miles of wire, not the fence. It means that if you use 3 wires(strands) in your fence over 3 miles, the total length of the fence will be 9 miles. So it would be helpful if you chose a fence charger that can output at least 9 joules.

Manufacturers claim that their fence energizers can cover 6-20 miles per joule, but these results are obtained under ideal conditions.

However, in reality, vegetation, dry soil, quality of conductors, and many other factors influence the power of your electric fence. So it’s recommended to obtain at least one joule/mile.

How to increase joules in a fence charger?

  1. Reduce vegetation as much as you can.
  2. Use an AC power fence charger.
  3. Use quality conductors in the fence wires and ground rods/stakes.
  4. Keep the soil moist.
  5. Resolve any short circuits.

As much as you make it easy for the current to move in the fence, you’ll require fewer joules(energy) to maintain power; thus, it’ll save power consumption.

Below, we’ve discussed the factors that impact the joules in a fence:

1. Area to be covered (length of wires):

First, we have to measure the area where you’ll install electric fence wires. Multiply the length of the area(in miles) by the number of wires you want to use in your electric fence. For a longer electric fence, use a charger to output more joules.

If your electric fence has multiple wires, the length of all wires is added, and your charger has to output joules according to the total length in miles.

For example, your electric fence has 5 wires covering 2 miles, meaning the total length of wires is 10 miles(5*2). So you should select at least a 10-mile fence charger that outputs a joule per mile of wire length. If your fence wires are 10 miles long, you’ll need a fence charger producing at least 10 joules. 

2. Grounding/Earthing:

For per joule output of the electric fence charger, you’ll need 3 feet of ground rods. If your fence energizer outputs 10 joules, you’ll need a 3×10=30 feet ground rod under the soil(ground).

As ground rods are required to catch free electrons that pass from fence wires to the soil through the animal body. For a powerful fence charger that outputs more joules, you’ll need more ground rods to maintain a steady charge.

If the earth is dry, there will be low current conductivity due to low water levels (moisture). Less water and rugged earth will make it difficult for free electrons to reach the ground rods(earth stakes). So, very few electrons will reach back to the ground rods/earth stakes, compared to moist soil.

So, power(charge) in the fence will be reduced, and your fence energizer will have to output more joules(energy) to maintain power in your fence.

3. AC vs DC fence chargers:

A fence energizer inputs voltage from an AC or DC power source and turns it into high voltage (3-12 kilovolts) with the help of a transformer. 

An electric fence charger designed for AC power(plug-in fence charge) can output more joules. AC power supply delivers high voltage and amperage, so there is always excessive current available to counter power shortage in your fence.

However, a DC(battery-based) or a solar fence charger has a limited current supply(voltage), so your fence charger may not be capable of maintaining a powerful charge under heavy loads, dry soil, and vegetation.

4. Type of animals to be kept:

Joule’s requirement in your fence also depends on the animals to be kept. For example, animals such as deer don’t have excessive hairs, so one joule per mile will be enough to deliver a solid shock.

However, some animals, such as sheep, are covered with wool that provides insulation and protects them from a memorable shock. So, to keep sheep in, you’ll require a high-power charger to deliver 3-6 joules per mile. Such high-energy fence will deliver a shock passing through their wool to skin.

5. Quality of conductors:

The quality of conductors used in the fence wires, ground rods, and jumper wires significantly impacts the power and joules in your fence. A poor or rusted conductor will make it hard to deliver(push) voltage(current) in the fence. So you’ll require a high-joule fence charger that can burn(deliver) enough energy to power the whole fence.

A good conductor will make it easy to deliver voltage pulse in the entire fence, thus requiring low energy(joules). So, it’s better to use a conductor in your electric fence with minimum resistance or excellent conductivity. 

The better the conductivity of the current, you’ll need fewer joules to power your fence.

6. Stored vs. Output Joules

Stored joule is the amount of energy saved in the energizer’s capacitor. Output joules are the energy sent through the energizer into the fence wires.

A capacitor is a device that stores charge in two or more conductor plates separated by an insulator.

Capacitors are used to create and store a potential difference. If the already installed capacitors are replaced with a higher-capacity capacitor, your fence charger will have an extra capacity to overcome power loss due to heavy loads.

Stored joules are always 30-40% higher than the output joules. For example, if a fence charger/capacitor stores 10 joules, the output will be around 7 joules.

How does a fence charger transform current?

Capacitors stores charge (current) up to a certain level and then discharge themself to pass current to the transformer. A transformer installed after the capacitors convert this low voltage & high amperage current to a high voltage pulse (in kilovolts) having very low amperage and pushes it into the fence wires.

But the output joules of the transformer are always less than the stored joules in the capacitors. Because transformers are not 100% efficient in converting current. Also, the capacitors don’t discharge completely, as some residue charge is always left in the capacitors.

It is essential to differentiate between stored and output joules, as some manufacturers fool their customers by labeling the stored joules as output joules.

Before buying, check the fence energizer output joule rating and ensure it meets all your requirements, such as covering the required length and output of at least one joule per mile.

So choose a fence charger that has a higher output joule rating than your requirements, as it’ll be beneficial if you tend to increase the fence length in the future.

Frequently Asked Questions

1: How many joules for cattle?

For cattle & livestock, maintain at least one joule per mile, but greater is better. However, animals covered with wool, such as sheep and horses, maintain at least 3-6 joules per mile to deliver a memorable shock. Bulls and bison require 5-8 wire fences, three of which must be electrified. 

2: How many joules for a dog fence?

Dog fences should also have one joule per mile, like other animals. When dogs see another animal, get terrified, or want to chase them, they can jump or cross the live fence wires, ignoring the pain of voltage pulse shock. So install your fence such that there is no extra gap for dogs to escape. Place the live(electrified) wires at the nose height. Also, keep in mind that dogs can jump up to 6 feet.

3: How many joules for a bear fence?

Bear fences also require at least .7-1 joule per mile. Bears are more prone to sense and touch electric fences. So, baiting the live wire in the fence with honey or peanut butter urges the bear to taste the wires, resulting in a notable shock.

4: How many joules for a horse fence?

For a horse fence, your fence charger should output at least 1 joule per mile, like any other animal. Install at least 5 5-foot high fences, as horses can jump high.


Finally, I would say joule is the heat generated when current passes through an electric fence. In ideal conditions, a fence charger can easily cover 5-10 miles or more with one joule of output.

However, most of the time, environmental conditions are not ideal. Vegetation, rusted or poor conductors, dry soil, and a low-power fence charger will have an extra load, hence requiring more energy(joules) to deliver current in the entire fence.

So, according to the recommended joules setting for a fence, you should maintain at least 1 joule per mile.

Hopefully, this explanation is quite enough to understand the basic concept of joules. If you still have any queries about what a joule is in an electric fence, ask us in the comments below.

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