October 30, 2014
Selecting an electric fence energizer for your farm
The particular model of energizer to choose depends on the total distance of fence to be charged.
Manufacturer's claims on this point can be very confusing. If the energizer has a joule rating assigned
to it, it should be simple to determine how many joules of power are needed for your situation.
However it is not so simple. One major fence company makes energizer recommendations based on
the premise of ½ mile of fence per joule of energizer capacity. Another manufacturer suggests up
to 10 miles of fence per joule of energizer capacity. The first company is presenting a worst-case user
scenario while the other is presenting a best-case scenario. Our experience has been that allowing
one mile of fence per joule output will give satisfactory performance in most situations.
The type and construction of the fence affects the total mileage capacity of the energizer. Single wire
paddock subdivision fences for cattle are generally well up out of the vegetation and, if well
constructed from high quality material, several miles of this type fence can be electrified for every
joule of energizer output. Multi-strand perimeter fences and three strand sheep paddock fences may
carry very heavy vegetation load on the lower wires and much less total distance will be served by
the same energizer as used in the single wire situation.
The next decision to make is whether to use a battery unit or mains unit. This choice depends mainly
on availability of 110v or 220v power source. In almost all situations where power is available, it
is advisable to use a mains unit. Mains units are invariably cheaper per unit of output power than
battery units. Worries about keeping batteries charged up and the fence hot are eliminated by mains
power. Monthly electricity cost for operating mains units, based on $.08/kWh, range from less than
$.05/month for small units to around $1.50/month on very high-powered units. Even if a mile of
feeder wire is required to bring power from a building housing the mains unit to the actual pasture,
it will generally be cheaper in the long run to do this rather than use a battery unit with solar
recharging of a battery. Available mains power units run from less than 1 joule output to 20+ joule
If mains power is not available and a battery unit is required, there are many sizes and types to
choose from. Self-contained lantern batteries, either 6 or 9-volt cells or a multiple of ‘D’ cells, may
power very small units with less than 1 joule output. New technology in dry-cell and gel-cell
batteries has increased the time period which battery units can be operated between recharges or
replacement. Gel-cell batteries are generally designed to be recharged but require special chargers
to maintain battery life. A new generation of long-life, disposable dry-cell batteries offers potential
for more than six months battery life. For remote units that are not visited on a regular basis, these
long life battery systems offer greater management opportunity than has previously existed without
use of solar recharge systems. Current technology in “smart” energizers allows for even greater
battery life. Smart energizers detect the load upon the fence and adjust pulse output to the demand.
If load is minimal, the battery draw is reduced and battery life is prolonged.
Larger wet-cell units up to over 20 joule output can be powered by 12v automotive type or deep
cycle gel batteries. When using 12v systems, it is highly advisable to install a solar panel to keep the
battery charged. The higher the output of the energizer, the more rapidly a storage battery is drained.
The largest wet-cell battery units may fully drain a deep cycle 12v battery in less than a week. To
avoid the weekly turnaround of batteries, a solar recharge system is a must.
There are also energizers available that can be used either as 12v battery or 110v mains units. These
units offer a great deal of versatility in use. Some graziers are concerned about controlling animals
if the power goes off for an extended period due to weather events such as hurricanes and ice storms.
The dual power units offer the opportunity to switch to battery power if the mains power is out for
more than a few hours. They can also travel from the home base of operations to a remote grazing
location to be used with battery. These units can work well for custom graziers operating on leased
Some people have the perception that battery units are inherently less powerful than mains units.
When comparing units with the same output joule rating, the capacity to energize a given length of
fence are the same, as long as the battery is fully charged. It is only when the battery has been
partially drained that battery units can be considerably weaker than their mains counterparts. Using
a solar or wind recharge system can eliminate the weak battery failure.