Anyone dealing with electric fencing will soon come across the term "joule". But what exactly does this value indicate? And what’s the difference between stored energy and output energy? At first glance, both values might seem similar, but in fact, they describe two completely different aspects. Understanding the difference helps you better assess the performance of an electric fence energiser.
An electric fence energiser initially stores energy in a capacitor. Once a certain energy level is reached, it is passed on to a transformer. The voltage is increased there, while the current is reduced. This conversion is essential as it ensures the pulse is strong enough to deter animals.
However, this process is not loss-free: Transformers in electric fence energisers typically operate at an efficiency of around 60 to 75 percent. This means that only part of the stored energy is actually delivered to the fence as output energy.
Example:
An energiser with 10 joules of stored energy and 60% efficiency delivers only 6 joules of output energy to the fence.
The formula is:
Stored Energy × Efficiency = Output Energy
In this case: 10 J × 0.60 = 6 J
Here’s a comparison for clarity:
Think of it like a car: the fuel tank shows how much energy is available, that’s the stored energy. What matters, however, is how much of that energy is actually used to drive the wheels, that’s the output energy. A large tank alone is no use if the engine converts it inefficiently.
Some manufacturers primarily highlight the stored energy, often without making it clear that only a portion of it reaches the fence. On paper, this makes their devices appear much more powerful than they actually are. This becomes especially misleading when comparing them with products that correctly state their output energy (output joules).
Important to know:
Only the output energy reflects what an animal actually feels at the fence. It determines how effective the fence will be, not the stored energy inside the device. While advertising stored energy is allowed, without proper context it says little about real performance.
A joule is the physical unit of energy. In the context of electric fences, it refers to the amount of energy released with each electrical pulse. The higher the number, the stronger the pulse, but only if it’s referring to the output energy.
Tips for choosing the right fence energiser:
✅ Always check the output energy (output joules)
✅ Don’t be misled by high stored energy figures
✅ Choose the right device based on animal type and fence length: Our Energiser Helper
✅ Reputable manufacturers state both values transparently