Electric Fence Energiser Calculator NZ
The mistake that sits behind most undersized electric fence systems is measuring the fence line instead of the wire. A kilometre of fence carrying three live wires is three kilometres of electrified wire as far as the energiser is concerned, and a farm that adds a second and third wire to existing lines can triple its load without adding a metre of fence. This works out the total electrified wire length across however many runs you have, then sets it against what your energiser is rated to drive and tells you what headroom is left. Note that it asks you for the energiser rating rather than supplying one. Energisers are rated in joules, but manufacturers quote their own kilometre figures against those joules and they differ substantially between makers, so a table of joules to kilometres taken from one brand would be wrong for another. Use the figure on your own unit or in its manual. The other thing worth knowing is that the manufacturer usually publishes two numbers: a headline figure for clean fence and a much lower one for fence under load. The second is the one to size on, because vegetation on the wires in spring is not an unusual condition, it is most of the year.
| Run | Length, m | Live wires | Electrified wire, km |
|---|---|---|---|
| Boundary | 6.4 km | ||
| Internal subdivision | 5.4 km | ||
| Lanes and temporary | 1.8 km |
The fence line measures 10.4 km but the energiser is driving 13.6 km of wire, which is the figure that matters. Size on the manufacturer figure for fence under load, not the clean fence headline. Vegetation on the wires is not an unusual condition.
How it works
Each run is its length multiplied by the number of live wires on it, which gives the electrified wire length for that run, converted to kilometres. Adding the runs gives the total the energiser has to drive. The fence line figure is shown alongside it deliberately, because the gap between the two is the thing most often missed: they are the same number only on a single wire system. The growth allowance inflates the total by whatever percentage you set, and the headroom is what your energiser rating leaves after that.
Where the power actually goes
Into the ground, mostly, through anything touching a live wire. Spring growth against the bottom wire drains power continuously across every metre it touches, which is why a system that held stock comfortably in August struggles in October without anything having been changed. Cracked insulators, a wire resting against a batten or a steel standard, and poor earthing all do the same thing. Earthing in particular is the most commonly undersized part of any electric fence system and the cheapest part to put right, and no amount of extra joules compensates for it.
Worked example
A farm with 3,200 metres of boundary carrying two live wires, 5,400 metres of internal subdivision on a single wire and 1,800 metres of lanes on a single wire has 10.4 kilometres of fence line but 13.6 kilometres of electrified wire. The boundary alone accounts for 6.4 kilometres of that from 3.2 kilometres of fence. Adding a 25 percent allowance for spring growth and future runs takes the requirement to 17.0 kilometres, which a unit rated at 20 kilometres under load covers with 3.0 kilometres to spare. Sizing on the fence line figure of 10.4 kilometres instead would have understated the load by 3.2 kilometres of wire.
Related calculators
- Farm Fencing Materials: posts, battens and wire for the same fence.
- Stock Units: what the paddocks are carrying.
- Lifestyle Block Stocking: sizing stock to a small block.
- Stock Water: the other thing every paddock needs.