Electric Fence Energizer Sizing Calculator (Joules, Wire & Ground Rods)

How it works

wire km = fence km × electrified strands · output J ≈ 0.6 J per km of wire (1 J/mile) × vegetation factor (1 clean, 2 light, 3.5 heavy) × animal factor (1.5 wool/predators, 2 netting) · stored (rated) J ≈ 1.4 × output · ground rods ≈ 1 m per output joule, ≥ 3 · battery Ah = daily Ah × days ÷ 0.5

An electric fence is a psychological barrier: the animal must receive a sharp, memorable pulse the first time it touches the wire, and the energizer's job is to keep the voltage high everywhere along the fence despite the energy that leaks away through grass, poor joints and the fence's own length. Joule ratings scale with the total length of wire — every strand counts — and are multiplied for vegetation, which is by far the biggest drain, and for animals whose fleece or hair insulates them. Half of all weak fences are not underpowered but under-grounded, because the circuit only closes through the soil and back into the ground rods, so rod length is sized to the energizer as well.

Worked example

A 1,200 m cattle paddock with three hot strands has 3.6 km of wire; with light grass contact that calls for about 4.3 J of output, so a 6 J stored-rating unit, with 5 × 1 m ground rods. Materials come to roughly 3,780 m of wire, 104 posts at 12 m and about 312 insulators. Off-grid, the unit draws around 190 mA — 4.6 Ah a day — so a 130 Ah deep-cycle battery gives two weeks and a 20 W panel keeps it charged in summer.

Frequently asked questions