Modern electric fence energizers have the capacity to maintain effective voltage for animal control
under much more adverse vegetative load conditions than earlier types. Low-impedance is the term
most commonly used to describe the modern energizer. Low-impedance energizers are characterized
by moderate voltage and a very short pulse length. Manufacturers have attempted to rate the power
capacity of fence energizers by several methods including miles of fence powered, farm acreage
covered, voltage output, effective voltage under varying resistance levels, and joules of energy.
Each of these ratings has its limitations.
Miles of fence powered is meaningless unless
the wire gauge, vegetation load, quality of
fence construction, and other factors are
specifically indicated. Size of the farm means
nothing without some idea of the level of
paddock subdivision on the farm. Voltage
output is generally peak voltage potential
with no fence attached to the energizer or
"no-load" voltage, as it is commonly referred
to. An energizer with 15,000 v no-load
potential may actually be considerably less
powerful than a unit with 5000 v no-load.
Joules are the most common means of comparing fence energizers. One joule is equal to an electrical
output of 1 watt per second. Comparison by joule output is only valid if the pulse lengths are similar
and if measured at comparable resistance levels. Most manufacturers rate their units based on output
energy. This is the amount of energy the unit consistently pulses into the wire. A few companies use
stored joules to rate their units. Stored joule measurements are usually 20-30% greater than output
2 joules so units cannot necessarily be directly compared across manufacturers. For example, a 10-
joule stored energy output energizer is equivalent to a 7 to 8 joule output energy-rated unit.
Very often the energizer model number itself tells what the energizer capacity is in joules. The two
units shown below are examples where the ‘M’ indicates they are mains (110v plug-in) units and the
numeral specifies the output joule rating. The M1000 has 8 times the output power of the M150. This
does not mean it produces a higher voltage on the fence line, only that it will maintain the power
level on six times as much fence.
With these points in mind, how does one go about selecting the proper energizer for a particular
situation? You need to consider how many miles of fence will you have, will it be single or multiwire,
what is the expected vegetative load on the fence, what type of livestock or wildlife are you
attempting to deter, and what is available as a power source.