How to Choose an Energizer for Your Electric Fence System

There are a lot of names that refer to the power supply unit for your electric fence system. I have seen or heard them referred to as: energizer, energiser, charger, fencer, shockers, zappers and many other names. These are all names referring to the energy source used to deliver energy to electric fences. They come in all shapes, sizes, colors and performance ratings.

For this article I will use the term energizer since I feel that name is the most self-explanatory. Choosing the proper size energizer can be somewhat frustrating and maybe even a little bit intimidating. It may depend on the integrity or knowledge of the salesperson you are purchasing it from. There is a lot of myth and mystery about energizers in general, but it doesn’t have to be that way. A little research and a few simple rules can help simplify your choices and provide for optimum voltage on your fence wires.

At the top of your list of priorities (or considerations) should be your power supply options. The three main choices here are: (1) Plug in units for 110V or 220V, (2) Battery Powered units, or (3) Solar Powered units.

  1. 110V plug-in mains power is generally the most cost effective, meaning that you will get more power for your dollar. Therefore, I recommend that IF you have a power outlet available – use it. In general, these are available in both 110V and 220V. As the units get larger, some of them offer units in 220V operation. Personally, I have never seen a lot of need for the 220V units and sometimes it can cause a problem with outlets and connections at the plug in point. The largest energizers for sale in the USA may pull a maximum of 50 watts. That’s equivalent running a 50 watt light bulb. None of them will actually cost you much to run and usually will equate to pennies per month.
  2. Battery power is necessary for locations where conventional electric power is not available. You will pay a little more for this energizer option, plus the additional cost of a battery. You may also consider the cost of replacing a battery every 3rd or 4th year. It is highly recommended to use a good deep cell marine type battery, which are designed to discharge more slowly and completely. Note that some of the larger battery energizers may require more than one battery.
  3. Solar power is another option for use with battery-operated energizers. Solar panels can be added to most 12V battery-operated energizers. It is possible to pay about as much (or more) for the solar panel as for the energizer. Larger solar panels (over 12 watt) will also require a voltage regulator, which are usually included in the price of the panel. Smaller panels (12 watt and under) should have a blocking diode, which is usually not included, but can be purchased at electronic stores. Large solar units may require more than one battery. In essence the solar panel serves to keep your battery(s) charged. If you have enough battery storage, under normal conditions they will operate fine with a week of cloudy weather. However, the larger battery energizers (under heavy loads) can run a battery down in a hurry. Many all-in-one prepackaged solar energizers with solar panels built-in are generally a maximum of .75 joules and usually less. Therefore, they are only designed to power a maximum of a few miles of wire. Generally, you will need about 10 watts of solar panel for each output joule of the energizer. Example: a 6 joule charger will need a minimum of a 60 watt solar panel. But, always check with the energizer manufacturer for their recommendations.

Don’t judge an energizer by its cover or the box it comes in. We live in an age of some pretty fancy packaging and boxing today. Do your research and learn how to compare the various choices and be able to compare them “apples to apples”. To simplify this I would recommend looking at the output energy and compare them from that basis.

There really isn’t an international industry standard for comparing energizers. Some are rated in output joules, while others rate theirs in stored joules. When you compare energizers – please try to evaluate them as “apples to apples”. Output energy is the amount of energy that is delivered to the fence. Stored energy is the amount of energy stored inside the energizer (in the capacitors). Output energy (that is delivered to the fence) is usually about 30% less than the stored energy. Information is definitely not consistent between manufacturers but try to avoid comparing one brands output energy to another brands stored energy. That will cloud the issue easily.

Another topic of concern here is the country that the energizer has been manufactured in. I have discovered over the years that different countries have different standards for testing and rating their energizers. For example: (the last time I checked) New Zealand uses a 500 ohm fence load in their testing while Europe uses a 5000 ohm load. In real world comparisons this relates as 500 ohms being a relatively normal fence load, while a 5000 ohm load would be more like a laboratory setting with virtually no grass or weeds in your fence. That is a major difference in how that energizer will work on YOUR fence on your farm or ranch.

Another modern feature is one that is referred to as “power on demand”. With this feature there may be multiple capacitors inside the energizer. Based upon fence load they may not all discharge at the same time depending on fence load. These energizers monitor fence loads then ramp up / down their power output as the load increases or decreases.

What is a joule? Good question, but hard to explain. It is a measure of electrical energy in British Thermal Units. It is something like 1 watt for 1 second, which doesn’t mean a lot to me. What IS important is how much fence 1 joule of energy will power. This is a very debatable subject and one that usually gets very confusing to most livestock producers. To keep this as simple as possible I would like to refer to Joules as the power that pushes the electric pulse (or shock) down the fence wire.

To keep this as simple as possible, I am going to go out on the limb here and say that I feel that 1 joule should power about 3 miles of fence. This will be different from what is printed on the box of the energizer as most manufacturers will claim that their unit will power something like 10 miles per 1 output joule. I personally feel that 3 miles per joule estimation will compensate for most grass loads and minor shorts, still giving you adequate voltage on your fence wire. I have also heard people recommend that 1 joule will power 1 mile of fence – and this would be acceptable in extreme cases of very heavy loading.

So how does that compute? What I am suggesting is that if you have 15 miles of fence you should divide that by 3, giving you an answer of 5. You would need an energizer rated at 5 output joules to power 15 miles of fence. So what size energizer should you actually purchase? My recommendation for this hypothetical situation would be to buy an energizer in the 8 joule range. This will give you adequate power, including the addition of some more fences in the future.
Impedance? Historically there have been and are three major types of impedance for energizers: (1) High impedance, (2) wide impedance and (3) low impedance. Of these different types, the high impedance ones, for the most part, have been taken off the market. These were the older “weed burner” types. They had long slow pulses and they did burn thru weeds, but they were also known to start fires and in general not be all that safe. Most modern day energizers are now what are considered low impedance. The duration or length of the electrical pulse is the dominant factor here. To be considered low impedance the duration of the pulse must be less than .003 of a second. You can also relate impedance to “leakage” meaning that with fence loading a high impedance charger will have more leakage of power and a low impedance charger will have low leakage of power. In general, those manufacturers that have designed their pulses to be more streamlined, bullet shaped and of short length will most likely have more power going thru vegetation and fence loading.

In non-technical terms, low impedance means the fence charger is designed to effectively shock through vegetation and other foreign matters contacting the charged wire. Vegetation such as grass, weeds, vines, etc., contacting the charged wire tends to impede or stop the flow of electricity by “grounding out” the fence. This is a common problem which exists on most electric fences. The low impedance fence charger is a solid-state capacitor discharge design and has enough power to force the shock through vegetation.

Basing your energizer requirements on acreage? Some manufacturers may also indicate how many acres a particular energizer will power. To me, this can be a poor estimate for power requirements. There are too many variables involved such as configuration of the acreage, amount of cross fencing, number of strands, method of connections, etc. Personally I would prefer to compute the distance of fencing rather than the acreage.

Poliwire: In general, poliwire products are intended for short distances of portable fence and ones that you will be moving from place to place. However, in some cases I have seen people with miles, upon miles, of poliwire in use. Poli products have much more resistance than say 12.5 gauge HT wire, due to the diameter of the conductors. You will normally need more power when energizing long distances of poliwire.

Strip Grazer Energizers: These are small energizers that are made for charging short runs of temporary fence, generally less than a mile in total length. Most of these energizers have output joules of 0.17 to 0.25 output joules. They work quite well for their intended use. However, if you choose to put them on longer distances, then don’t expect them to have a lot of voltage and power over longer distances.

Consider keeping a spare energizer on hand. Keep in mind that this is an electronic device and may need worked on at some time. When you are putting all your confidence into an electric fence system – you need voltage on the fence wires. I suggest that it is a wise decision to keep a spare energizer on hand. Or, ask your local fence dealer if they offer loaner units while yours is away from the farm for service or repair.


About Gary Duncan

Gary has been active in the fence business for over 15 years. He also raises Highland cattle in a management intensive grazing system and was the first person to market the PasturePro post back in 2005. He enjoys discussing all things grazing and is the main contributor to the blog.
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