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.
This entry was posted in Buying Tips, Fencing Tips. Bookmark the permalink.

24 Responses to How to Choose an Energizer for Your Electric Fence System

  1. Thank you so much. I have been going through a tough learning curve with energizers and electric netting for sheep and this article was one of the few helpful ones.

  2. avatar Tuliraba Fredrick says:

    I intend to constract a 4mile power fence with power source as solar. How many Energisers do i need.

    • avatar Steve Freeman says:

      Hi Tuliraba,

      The question of how large or in your case how many energizers is always good for a debate among fence installers. And like many questions the answer is “it depends”. The reason for the qualifier:

      How many wires will be electrified in the fence?
      Humid region-high weed load or dry arid country with little weed load?
      What kind of wire–12.5 high tensile or poli-wire?

      But whatever the answers to those questions you can get by with one energizer. Solar is simply a way to charge the battery to run the energizer and you can get large solar panels to keep batteries charged that will power
      large energizers. Here’s a nice compact unit one of our dealers sell made by Gallagher
      Solar panels have gone down in price from a few years ago so you can power even the largest units at a fairly reasonable price. And just to throw in my standard advice, if it’s at all possible you can always buy a lot more power for the buck if you can find a way to use a 110v plug-in unit instead of a charge using a solar charger.

  3. avatar Kevin Kugel says:


    I was wndering if you can answer a few questions for me:
    I have stubborn dogs that keep climbing the kennel fence and getting out. I want to electrify it to train them to respect the fence. I have no 110 power available so I wanted to go the battery route.

  4. avatar Kevin Kugel says:

    My questons are:
    1. Is the sears model 436.77670 6 volt battery fencer safe? It was given to me by my father in law when he cleaned out a barn and it looks pretty old.
    2. When I hook up a 6 volt battery to it it makes a clicking noise like it is pulsing. Is that normal?
    3. How long can I expect a 6 volt battery to last if I leave it on all of the time?

  5. avatar Paul DeWinter says:

    Your web site was a helpful start to my project – build an electric fence around the solar pool heater on my roof. Squirrels like to chew on the small heater tubes and it cost me $149 +parts every time they eat a hole in it. I’m looking at adding an electric fence about 1 foot high around the solar panel’s area to shock or fry the little rodents. Plan is for alternating ground and hot wires every 2 inches mounted on insulators which will mount to the 1.5inch diameter inlet and outlet pipes of the heater. It will border the entire panel area. Has this ever been done before?

    Best Regards,

  6. avatar jim bellizzi says:

    I have 5 strands about 700 ft. long each. animal is a 16 lb. cat with medium long hair.
    I would like to keep him in yard without hurting or injuring him. currently using an old zeraba which they tell me puts out about 1200 watts. THE CAT WALKS THROUGH FENCE AS IF IT IS OFF. DOGS YELP IF TOUCHING IT.

  7. avatar mike says:

    What will it mean if you buy an energizer with a higher output rating than you require eg your calcs say get one for a 5 acre property but unit you buy on special is good up to 30. Will this give more ‘shock’ i.e. I don’t want to kill an animal.

    • avatar Steve Freeman says:

      Hi Mike,

      Good question-one I think many people think but don’t ask. Within reason, buying a energizer that’s rated for more acreage than you are powering is not a problem, in fact, it’s usually recommended. Acreage recommendations are not very accurate and should be read with a raised eyebrow. While some manufacturers question joules ratings, joules output readings are the most accurate measurement I know of to rate the shocking power of an energizer. And to make comparing more complicated some companies measure the chargers by acreage and others by miles. While a miles rating is still a little misleading it is more accurate than acreage.
      In my experience a more powerful energizer shouldn’t have a much higher voltage coming from the energizer but instead should have the capability to “push” the voltage [shocking power] through more fence and load [grass, brush] than a less powerful energizer. In other words, on a perfectly clean [no load] five acres of one wire fence the voltage readings from an energizer claiming to be good for five acres should have close to the same voltage reading as an energizer rated for thirty acres. You are probably looking at energizers that have joule ratings between 1-3 output joules-neither of which should have high enough shocking power to harm your animals.

  8. avatar robin says:

    i appreciate the information.
    I am looking to run hot wire between the outside runs on my barn and additionally want some to put around an enclosure for my miniature horses and goats. The whole length I am fencing is around 200 feet so not a huge area but I want to make sure fence is strong enough to give horses the message and to keep predators away from my goats. any help is appreciated

    • avatar Steve Freeman says:

      Hi Robin,

      Thank you for the e-mail. Horses and goats make for an interesting but tough combination. It’s fairly easy to fence horses with electric fence but it has to be done with safety in mind. When fencing goats it’s always a challenge and electric fence is the best but often means more wires or woven wire which adds to the cost-especially when needing to use horse safe fence.

      Two suggested styles of fencing–one to ensure the complete safety of your horses and one to ensure effective predator control for your goats.

      For the horse safe fence I would suggest you build a braided rope fence. Since you are raising miniature horses your top wire does not have to be as high as with conventional horse fence but because you are also trying to control and protect goats it will require more strands down low. Here’s a link to one of our articles about building a fence with braided rope. The only difference for your fence is adding more strands to the lower part of the fence.

      For complete predator proof but not completely horse safe I would recommend building a fence with high tensile woven wire on the bottom and electrified braided rope on top [visibility]. With the short run you are building you can electrify the woven wire which will make the fence almost impenetrable for predators and goats. Here is a link to the high tensile woven wire one of our dealers carry–
      The worry about woven wire is a horse getting a foot through and caught in the one of the squares. If your horses are respectful of fences and you don’t have problems with dominant horses running other horses out of the pen you may feel confident with this method of fencing. It is an individual decision.

      Hope these two suggestions help a bit and please write back if you would like to discuss either or others more.


  9. avatar joy says:

    I currently am using a solar powered fence charger. i am now having a problem with my hogs getting out now that they are bigger. Will getting a 120v AC charger with say…. 2 jules be more effective at keeping them in? Is it the increase in jules that make a more powerful shock?
    thanks joy

    • avatar Steve Freeman says:

      Hi Joy,

      Unless you have a very large solar powered fence charger, then yes, a 2 joule charger will be more powerful. When you increase your joules make sure you have the needed amount of ground rods to allow the charger works to it’s capacity. You will always get more bang [joule] for your buck using a 110v charger in comparison to a solar charged battery system. Though solar panels have come down in price over the years, it still adds substantially to the cost of a unit. An AC unit will usually burn about the same amount of electricity as a 60 watt bulb, so if you can use AC it’s usually best.

      I recently had a customer call with the exact same problem you are having with your hogs. I asked him to check the ground and the connections-which were all okay. He couldn’t get the meter to read over 2.5v, even when we he disconnected the positive wire from the charger and his ground was not showing any voltage meaning there was not a dead short on the fence. I suggested to change the battery on the charger.

      Remember a solar powered charger is really just a battery charger with a solar unit added to charge the battery–and batteries do get old. Instead of buying a new battery he swapped chargers [identical chargers] and found he jumped his voltage to 5,000v with the other charger. But that one also faded after a couple of days. We think it may be that it’s been so cloudy and overcast recently that the batteries are not being charged enough. He was going to purchase a 110 unit–and try to get the hogs back in the pen… Fencing hogs and goats is more difficult than cattle. Cattle rarely check to see if the fence is hot–goats and hogs check often!

      Questions for you:

      Do you have a voltmeter and if so what voltage is it reading?

      What kind of wire are you using and where in the fence are the hogs getting out [under the bottom wire or through the wires?]

      What are you using for a ground system?

      Thanks for your question,

  10. avatar Michael says:

    Hi Steve,
    Thank you for posting all of this great in info. I am considering putting up an electric fence for a few pigs this spring. This will be our first run with electric fence as well as with raising pigs. Your writings make good sense to me, however, the questions I have are related to the distance between the charger and the fence line;

    How close does the charger need to be from the fence line? My closest elect outlet in relation to the fence is about 100ft.
    Is there a way to run this 100ft under ground without compromising the function of the system?
    Thank you in advance, I sure appreciate you answering all your readers!

    • avatar Steve Freeman says:

      Hi Michael

      Thank you for your e-mail. I really enjoy talking fence–but be warned that I’m not an expert on chargers just have farm experience using them over the last 30 years. One hundred feet is no problem, but depending on the distance of fence you plan to charge I might suggest actually running two insulated wires underground to the fence. Two wires will give you more power to the fence, or, if you ever decide to go with a hot/cold fence it gives you one wire all ready buried and threaded through your conduit to connect back to both the ground and output side of your charger. Do use conduit for the 100feet as it seems no matter how high a quality an insulated wire you buy the possibility of getting a short through the insulation is there. You might find it easier to push your wire through that length of conduit easier if you thread it through the 20′ sticks of conduit before you glue it together. One hundred feet is a long way to push the wire and if you don’t glue them first you can keep thread one 20′ section at a time.

      hope this helps and again thanks for the nice words.


  11. avatar Dean says:

    Could you explain why 5000 ohms resistance is less load than 500 ohms resistance?

  12. avatar ron says:

    I am fencing in horses. I have 5 ac. Dimensions are 666 ft x 333ft. I am running two wires. I purchased but have not installed a Zareba 50 mile which is suppose to be 2 joules. I thought this might be to much 50 mile since the total lengeth of both wires is just shy of 2,000 ft. Any thoughts on the proper size chaarger to use?

    Also, I read where you mentioned about running 2 wires to the fence. I only have about 20 ft to run but would it be worthwhile to do?

    Thank you

    • avatar Lacy Weimer says:

      The Zareba 50 mile charger is NOT to big. Zareba advertises this as 50 mile, and that would be correct if your fence line was absolutely perfect. Meaning there where never any weeds or voltage leaking and that is very rare. This is truly about a 2 mile charger on the typically fence line. This energizer will work well! With regards to the 2nd question-Not sure I understand exactly what your asking…You will want to run one strand of underground from your positive energizer terminal to the fence line. Then jumper your two fence line wires together so that both are energized. Please post again if this did not answer your question.

  13. avatar Sharon says:

    The Zareba® 10 Mile AC Low Impedance Charger can you tell me about how much electic this will use in a month time.

  14. avatar Aaron Hall says:

    I plan to build a pig pen but am apprehensive as to whether or not my current charger will contain them. I have a .1 joule solar charger, and from my math after reading the 3 mile per 1 joule rule. My plans for a 280 ft (wire footage) would come to .05 mile of line, so if I divide the .05 by 3 it gives me a requirement .017 joules to successfully charge the pen. Is this correct? The pen itself will be 40×60 with 2 cross fences creating 3 20×40 pens for the pigs to be rotated amongst.

    • avatar Lacy Weimer says:

      I would air to the side of caution and use at least a 1/2 to 1 Joule unit on a pig pen.
      Your math is correct, but more power is better in case of resistance and fence line failures.

      • avatar Aaron Hall says:

        ok I think I decided to err to the side of caution and run a standard woven field fence with the electric run on the inside as an escape deterrent .

  15. avatar Bob says:

    One in the final four, and none in the stink: Shockers are people who harvest wheat, apparently. That being said, what the hell is with the mascot? Is it a

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