Electric fence chargers: joules, miles, and which number lies

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Research. We have not run this one through a winter yet. What follows is how chargers are rated, what those ratings actually mean on the ground, and how we are sizing ours. When it has been in service long enough to say more, this page gets updated with the date.

Every electric fence charger is sold on two numbers, and one of them is close to useless.

The number on the front of the box

Miles. "Powers up to 25 miles of fence."

That figure comes from a test with a single strand of wire, no vegetation touching it anywhere, and perfect ground. It is not a lie exactly. It is a laboratory result printed in a font meant for a barn.

Real fence has grass on it. Real fence has three strands, or five. Real fence runs through a low spot that stays wet. Every one of those pulls the number down, and the box does not mention any of them.

The number that matters

Joules. Specifically stored joules and output joules, which are different, and the second is the honest one.

Joules measure the size of the pulse the charger sends down the wire. That is the thing that has to make it past the wet grass and still be convincing when it reaches an animal.

Rough working figures, for a small place:

• 0.5 joules — a few strands, short run, kept clean. A garden fence, a temporary paddock.
• 1 to 2 joules — the usual answer for a smallholding with a few acres and some vegetation contact.
• 4 joules and up — long runs, heavy brush, or animals with an opinion about fences.

Buy up. A charger that is too big costs a little more and does nothing worse. A charger that is too small produces a fence that works in April and stops working in June when the grass comes up, which is exactly when you stop trusting it, which is exactly when the animals learn.

Mains or battery

Mains, if you can reach it. It is cheaper per joule, it never needs charging, and it does not care what the weather is doing. Run the wire to the fence rather than the charger to the paddock.

Battery or solar where you cannot. There is a separate piece on solar chargers, because the sizing rules are different and the failure mode is seasonal.

The thing nobody sells you

The ground system. A charger is only half a circuit. The pulse goes out along the wire, through the animal, into the earth, and back to the charger through your ground rods. If the ground side is weak, the fence is weak, and no amount of charger fixes it.

The usual guidance is three ground rods, six to eight feet, spaced ten feet apart, clamped with proper clamps. Most people driving their first fence put in one short rod and wonder why the fence tests fine at the charger and does nothing at the far corner.

It is the single cheapest improvement available to most fences, and it is the part first-timers skimp on — us included, on paper, until we read the spacing guidance twice.

What we are buying and why

The rule here is settled and the purchase is not. Size on joules rather than miles, buy up rather than down, three rods rather than one — the page has already argued all of that. What is missing is the number itself, and the number waits on a finished line: a real strand count, a real length, and an honest look at how much grass will be lying on the wire in June. The ground rods belong to the same purchase, and none of them are driven.

When a charger is wired in, this section gets the model, its output joules, what it and the ground system cost together, and the date.

We will come back to this page after the first summer, when the grass has had its say. The Research tag at the top waits longer than that. It stands until this one has been through a winter here.

Before you buy

• Count your real strands and your real length, then size on joules, not miles.
• Add the ground rods to the budget now, not later.
• Buy a fence tester at the same time. Guessing whether a fence is live is a bad habit and there is a whole page on that.

— Chris D., McEwen, Tennessee