The Origin of Harmonics & Their Effects on a Power System

To understand the topic of this article (THD or Total Harmonic Distortion), we must build up our knowledge from the very basis of an electrical circuit.

A basic electrical circuit is composed of a voltage source and a load, which draws current from the source. This set-up operates with two basic electrical quantities; the current, and the voltage which can be either direct or alternating.

In case of the Direct Current or DC, the current flows only in one direction and has a constant value throughout time. In Alternating Current or AC, the current changes continuously and cyclically with time and reverses polarity after every half cycle. Therefore, the value of AC over time can be plotted as a waveform, most commonly a sine wave.

However, in electronics, we can never achieve a pure sine wave. Every wave has some sort of disturbances that distort its original waveform. These distortions are explained in terms of Total Harmonic Distortion (THD).

What Are Harmonics?

A harmonic is a sinusoidal component of a non-sinusoidal wave (e.g. the “real” waveform of an alternating current, at a frequency that is an integer multiple of the fundamental frequency. In other words, they are the components of the resulting wave which causes its non-sinusoidal shape.

A harmonic whose frequency is exactly equal to the original wave frequency is called a fundamental harmonic and its frequency is called the fundamental frequency.

The harmonic which has a frequency ’n’ times of the fundamental frequency is called an nth-order harmonic.

For example, a wave which has a fundamental frequency of 60 Hz will have its 2nd harmonic at
120 Hz, 3rd harmonic at 180 Hz, 4th harmonic at 240 Hz and so on.

Harmonic Components of a Fundamental Wave

Harmonic components of a fundamental wave

Harmonic components are present in a system at multiples of the fundamental frequency. Each component contributes towards the distortion in the original waveform. The cumulative effect of these harmonics on the original waveform is termed as Total Harmonic Distortion (THD).

Definition of Total Harmonic Distortion (THD)

Total Harmonic Distortion (THD) is the degree to which a current or voltage waveform is distorted. Mathematically, it is the ratio of the sum of values of all the harmonic components to the value at the fundamental frequency, expressed as a percentage.

How Do Harmonics Alter the Waveform?

Harmonics are sine waves with frequencies which are multiples of the fundamental frequency, which can interact with the original sine wave. Whenever two or more waves interact, they undergo a phenomenon called interference.


When two or more waves interact, they superimpose to form a resultant wave that has either a greater, lower or same amplitude. This phenomenon is called interference.

Wave interference (opens in a new tab) is of two types; constructive interference and destructive interference.

Constructive Interference

constructive interference

Constructive interference (source)

Consider two identical waves that are completely in-phase with each other.

This means that the crests of one fall on the crests of the other, and the troughs of one fall on the troughs of the other.

The magnitude of both waves gets added. Thus, the resulting wave has a greater amplitude than the two individual waves.

Destructive Interference

destructive interference

Destructive interference (source)

Consider two identical waves that are completely out-of-phase with each other. This means that the crests of one fall on the troughs of the other, and the troughs of one fall on the crests of the other.

The magnitude of both waves gets subtracted. Thus, the resulting wave has a lesser amplitude than the individual waves.

Practically, a pure constructive or destructive interference is very improbable. Thus, the result of wave interactions are a combination of constructive and destructive interferences.

Each wave has multiple harmonics. Out of these, some are in-phase and some are out-of-phase with the original waveform. The points, where the harmonics are in-phase with the original wave, are added up constructively. And the points where the harmonics are out-of-phase with the original wave are added up destructively.

Thus, the result is a distorted non-sinusoidal wave.

Wave Distortion Caused by Harmonics

Wave distortion caused by harmonics

Why Are Harmonics Produced?

There are two types of harmonics present in an electrical system; The current harmonics and the voltage harmonics. We will discuss each of them one by one.

Current Harmonics

The AC current in a power system has a near perfect waveform. The waveform gets affected whenever a load is connected to the system. Generally, there are two types of load; linear loads and non-linear loads.

The loads which draw current in a sinusoidal waveform are called linear loads. Current at any instant is proportional to the applied voltage. e.g. heaters, incandescent lamps, etc.

The loads which draw current in a non-sinusoidal waveform are called non-linear loads. e.g. computers, printers, microprocessors, fluorescence lightning, etc.

These non-linear loads are responsible for the distortion of the original current waveform. The harmonic components of the distorted current waveform are called the current harmonics.

Voltage Harmonics

The current harmonics, in turn, are responsible for the voltage harmonics. However, the effect of voltage harmonics depends upon the impedance (resistance) between the voltage source and the load.

According to Ohm’s law (opens in a new tab), a current flowing through a resistor creates a voltage drop. This voltage drop is always proportional to the current. Since the current is non-sinusoidal, the voltage drop will also be non-sinusoidal.

Thus, the voltage waveform gets distorted. The harmonic components of this distorted voltage waveform are termed voltage harmonics, which are proportional to the impedance.

Effects of Harmonics

1. Transformers Losses

Power losses in a transformer affect the overall efficiency of the power system. Therefore, their maintenance is of great importance.

Hysteresis Loss: Hysteresis (opens in a new tab) loss is due to constant magnetization and demagnetization of the transformer core. This happens because the AC supply reverses its direction after every half cycle.

Eddy Currents Loss: Eddy currents (opens in a new tab) losses are due to the circulating currents in the transformer core. These circulating currents are produced due to the flux leakages in the core.

Hysteresis and eddy currents losses both depend upon the supply frequency. Hysteresis loss varies linearly with the frequency. Whereas, the eddy currents vary with the square of the frequency.

As higher-order harmonics have a very high frequency, they increase both the hysteresis and eddy current losses.

2. Motor Losses and Vibrations

Like transformers, motors are also affected by harmonics. The effects of hysteresis and eddy currents on the motor are the same as in the transformer. Both these losses are produced in the iron core of the motor. These losses increase due to the presence of higher-order harmonics.

The positive sequence harmonics develop magnetic field and current in the same direction as the fundamental voltage. Whereas, negative sequence harmonics develop the magnetic field and current in the opposite direction.

The interaction between these positive and negative sequence harmonics result in severe vibrations. In some cases, these vibrations may even damage the motor shaft.

3. Unbalanced Neutral Currents

In a three-phase power system, the current in each phase is equal in magnitude, but has a 120⁰ phase shift. Ideally, the currents in the three phases add up-to “zero”. This means that the neutral wire will carry no current.

But, due to the presence of harmonics, the current does not add up to zero. Under this condition, the neutral wire will now carry current. This neutral current is highly undesirable in a power system.

Moreover, the neutral wire generally has a small diameter and in turn a lesser current-carrying capacity. In such a case the neutral current will also cause overheating of the neutral wire.

4. Skin Effect

Skin effect relation with frequency

Skin effect relation with frequency

Skin effect (opens in a new tab) is a scenario in which the current does not flow through the whole conductor, but only through its outermost portion. Thus, the usable area of the conductor is reduced.

Skin effect happens due to the flux distribution in a conductor. Flux density is greater at the center and lesser near the edges of the conductor. The natural tendency of current is to flow through an area of lower flux density. Thus, the current moves towards the edges of the conductor.

The flux density is directly proportional to the supply frequency. As the higher-order harmonics have a high frequency, flux density also increases. The increase in flux density further reduces the area for current to flow.

5. Damage to Sensitive Electronic Equipment

Electronic equipment is, by nature, a non-linear load and thus, responsible for creation of harmonics, which have a negative impact on the power system. Vice versa, harmonics also adversely affect the electronic equipment.

IEEE Standard 519-2014 (opens in a new tab) advises on the permissible harmonic distortion limit for electronic equipment. As per the article, desktop computers and allied equipment like programmable controllers typically require AC sources that have no more than a 5% harmonic voltage distortion factor, with the largest single harmonic being no more than 3% of the fundamental voltage.

Harmonics above the permissible limit can cause malfunctioning of the electronic equipment. A higher percentage of harmonic distortion may even burn sensitive components.

Another phenomenon caused by harmonics is voltage notching (opens in a new tab). Voltage notches can also cause malfunctioning of sensitive electronic equipment.

Harmonics in a Portable Inverter Generator

To elaborate on the concept of clean power (as in sinusoidal, not to be confused with green energy), let us compare portable inverter generators and conventional generators in terms of the THD (Total Harmonic Distortion) produced.

The harmonic distortion in conventional generators is generally in the range of 9–10%. For some generators, it can be even higher than that (manufacturers will often say “under 25%”, as it also depends on the load). Due to such high distortion, these generators may not be suitable for circuits which contain dektop computers, routers, TVs and other sensitive electronic equipment. That said, since THD varies with load, some conventional generators may produce a clean enough power at lower loads. It is also worth noting that conventional generators with low THD exist, but are somewhat of an exception.

Contrary to this, a portable inverter generator’s distortion is in the 3–5% range, sometimes even lower. This range is well within the permissible limits of IEEE std 519-2014. Therefore, the electricity generated by portable inverter generators is safe (clean) for desktop computers, routers, TVs, etc.


Harmonics in a power system are caused by non-linear loads and most of the loads these days are non-linear. Therefore, as much as these harmonics are undesired, they will still be present in the power system.

However, these harmonics won’t cause trouble if the distortion level is within the permissible limit. See IEEE std 519-2014 for details on permissible limits for different kinds of loads.

Harmonics are a very technical subject and should only be dealt by an experienced person. Therefore, in case of any trouble, we would highly suggest you consult a professional.

Images of constructive and destructive interference: Picture by Haade, Wjh31, Quibik / CC BY-SA / Original modified to label crests and troughs



Manager & Editor of Early retired from the OPE industry, living in South Carolina. He now mostly spends his time traveling and taking care of his wife and grand-children.

  1. Good article, thank you for post it!

    Question: If a generator brand specifies a THD of around 8% at half load only. That means the THD could be worst at full load?

    P.S. I already asked for the THD at full load but them answered that it was not measured.

    Thank you,

  2. I used a 6500 watt Coleman gen on transfer switch during hurricane Ian and It damage two ceiling fans and one of two refrigerators . We never had time to turn on tvs or computers but my next generator will be an inverter, it’s the sure way to go.

  3. After suffering many days without power after Eileen and Sandy, I had an electrician put in a transfer switch and bought a Briggs traditional (non-inverter, 7k running) portable generator. At that place, I only lost power twice in the nine years that followed, and only for a few hours. I used that generator without any issues, and may have been lucky my laptops and PCs (I work in IT) had no issues during the working hours.

    When having my new place built, I tried to put as many of the high-heat-watt-pulling appliances on propane instead of electricity. So my heat/furnace, stove/range, dryer, and water heater are all propane, and I had a 30 amp inlet put in on the side of the house ready to go day one. After all the studies of THD and explanations, like the wonderful Paul has given here, I bought an inverter generator with the same output (7k running/8.75k peak). I can literally run my entire house without compromise on such a low output generator, although I haven’t tried running my dishwasher, toaster or sous-vide appliances at the time everything else is up and running. And the most astonishing thing of all this, is that I can run my 2.5 ton AC unit (small place, only 1500 sq/ft) on this generator, too, because of a soft start device I had an electrician install.

    With all my laptops and PCs, I just didn’t want to risk losing any electronic hardware, peripherals and data because of my job. With SOOOOOO many appliances being “smart” these days (like we need a smart washing machine and a smart refrigerator to hook up to the internet, really??? ) and having so many sensitive electronic components, I don’t understand why anyone would risk using non-inverter generators or generators with higher THD. I do believe people may not understand this risk as well as they need to when preparing for emergency power.

  4. Hi, I ran an old open cradle honda 3kva generator to keep us going through the many regular power outages in rural Ireland. It runs lights kettle and power tools etc great, but very quicly burned out satalight tv box, phone chargers, laptop chargers, the main circuit board in our fridge freezer and every usb outlet in all of our wall sockets. Therefore dont be fooled by people saying a standard generator will work fine.It might or it might cost you a fortune in repairs!
    I then bought a Sealey 4.5kva inverter generator, which just out of 1st year warranty and only used four times, refuses to run for more than a couple of minutes. I am advised it is an expensive inverter repair/ replacement. It has never activated the overload cut out or low oil cut out, so has no excuses. Just another piece of junk. Maybe I have to spend thousends on a good ,big generator?? Its a gamble! But you cant rely on the national grid out here to keep the freezers going, so no choice.
    Listen to this guy, he knows what hes talking about. Correct Wave forms and correct harmonics etc are crucial to modern gadgets / appliances.

  5. I have 2 Westinghouse 9500 Dual Fuel models, and they run everything just fine. Microwave ovens, TV’s Desktop computers. Not a single issue with either of them.

  6. Westinghouse now advertises a low THD generator that’s not an inverter, e.g. WGen12000 Generator. It has less than 5%.

    I guess they listened to the consumers. Which is extremely rare these days.

  7. Paul, the Best explanation of THD I’ve read. Portable generators have been used around here for a long time. I live in Michigan and we see power outages often. When shopping for portable generators most manufacturers tell you (in their ads) “combined with an L14-30R outlet, it’s Transfer Switch Ready for total household power. Not explained or referenced well is the THD. Buyers “do your homework”. Come to this site and heed Paul’s advice/information. You will make a more informed purchase when you’re ready to buy.
    With the advanced use of AC/furnace, Smart TV’s, dishwashers, etc. the need in “clean” sine wave alternating current appears to be more important than ever. I’m one of those that gets the call cause “my power is out and I need heat and my generator is not working”. The most recent call was a friend who has a 90% efficiency furnace (recent install) and has a Westinghouse 5300/6600 portable generator. After some research and lots of time, I found his new furnace/AC doesn’t like a THD of around 23%. I patched in a Ryobi inverter/generator to get heat. Westinghouse is aware of this and said we needed to purchase and install their “inverter board” for $200 to solve the problem. And they are out of stock. Great article Paul. Explains so the everyday person can make a wiser choice.

  8. Singe stage inline passive LC filters are a remedy to THD on power lines, yet it’s hard to find test data and sizing to clean up a cheap Gen sets power enough for some household electronics. Might be something worthy of a test and let the data drive it’s possible value.

  9. Jim, very nice job of spelling out THD based on generators. I just got off the phone with a major manufacturer and they could not tell me the THD max of one of their inverter generator models :(, just a generic “less than 6% means clean power”. Also, I checked my TV manual, and computer manual, and saw nothing related to maximum allowed power source THD. There a lot of folks that get by with using open frame ‘dirty power’ gens after hurricanes and I have never heard someone say it fried their electronics, but still, I want to not harm my electronics, but am finding it hard due to manufacturers of both gens and electronics not disclosing THD figures. Whats your thoughts please? Thanks.

  10. Is there any way to have a 10kw to 12kw portable generator that is considered safe for electronics?

  11. Just took delivery of the Generac 5735 17500w model. Hopefully it doesn’t fry anything on me… THD is around 20%

  12. Just wanna mention that every electronic out there does not run strict on 120v. Computers have a transformer that converts 120v ac to 15v to 20v ac. It seems to me this will be your filter. Phones charging at 5v with help of wall chargers with is step down transformer. Refrigerators run compressor 120v and have step down transformer for smaller more sensitive electronic. So I think THD is important but not a major deal for typical consumer.

    Not all electronics are same but I have not seen a computer’s motherboard run in 120v. Or a ac thermostat run 120v all them have a step down transformer.

    • Hey Alberto,

      Did you ever obtain any information corroborating your assumption re AC adapters providing protection against THD’s in excess of 5%? I believe that your question or point is absolutely key and am trying to get past the superficial and blanket statements that you shouldn’t run any sensitive electronics including laptops on the typical portable power generators.

  13. Paul, great article and explanation! It pretty much saved me from potentially costly errors!

  14. Thanks for the great information. Also, thank you for answering our questions. I know it takes time to break things down for us layman to understand. I have a similar situation as somebody else’s previous post running the heater from my Westinghouse WGen9500. I installed a new heater system and I’ve tried running it through the transfer switch as well as directly to the generator. The old system (less electronics) worked fine. Oddly my smart ups doesn’t like to allow the power straight through either (it’ll charge itself or power the heater from battery, but not both, when powered by the generator). So I’m toying with putting in a SolaHD 23-23-220-8 ( Am I right to assume this cvs conditioner, with its harmonic filtering, would clean up (lower to acceptable levels) the THD from the generator and possibly run the heater unit?

    • You’d have to ask SolaHD. From what I can see, the product you are mentioning is rated for 2000VA, which pretty much means 2000W.

  15. Good discussion. I’m way overdue to correctly address THD and individual harmonic distortion. Lots of power issues here. I’m covered on brownouts and surge EMI/RFI primarily at POU. Its the gen output 8K/12K B&S at measured ~15% that scares me. I converted it to tri-fuel and using Nat Gas after Sandy. OOP for 2 solid weeks. Had enough gas until the last couple days. Never again. Very nice unit other than that.Not seen another gen at any price with that kind of start wattage. Double the typical 20-25%.Is there a down stream inverter AC to DC to AC that I can add to the system. I do not want to us batteries. I gather it will be pricey but……..

    • You could get something like a double-conversion online UPS, which will first convert AC to DC, then DC back to AC. But for this kind of wattage output, the price would be ridiculous. It would be cheaper to just get an inverter generator.

  16. There are thousands of these Westinghouse units out there. Most receive very good consumer ratings from their owners. If these units were damaging expensive computers and electronics, you would be hearing about it. That is not the case. So the question now becomes, why aren’t we?

    • Any generator damages to electronics may take repeated hits over a long period before failure. When it does fail, assuming caused by generator usages, the user may no longer associate the failure to the generator because it was not an immediate failure on first use of the generator on the failed electronics. Hence, such genny damages were not reported as such. Just my theory. Also, those customers might have individual surge protector on the electronics, which is something different from THD, not sure.

  17. July 2020, bought a Westinghouse WGEN5300DF. Ran a toaster fine. Hair dryer outstanding. Microwave oven closed down after 5 seconds. That’s when I suspected “erratic” voltage. Needless to say, this is when the great awakening happened, THD. Westinghouse tech said “oh yes that’s a THD problem, 23%. Generator back to Sam’s. Have been ploughing through web page after web page and there are supposedly some less than 6% THD generators out there that are not inverters. I would like @6KW generator. Narrowed it down to about three companies. Will call them and ask. Wish me luck.

  18. Wow! That was precise. I was just about to buy a certain generator…and this article changed my mind. I would of ruined all my electronics. Thank you for taking the time to explain this.

  19. Why do you advertise a Westinghouse wg7500 on your website when it has a thd of less than 23 per cent I just bought one and now I am reluctant about using it

  20. Great article Paul, very informative. What product, if any, can be used to clean-up the sin wave and reduce the THD to acceptable levels on a standard generator? I’m assuming this would be something between the generator and the sensitive equipment load.

    • You’d be looking at something like an AC regenerator, or an on-line double-conversion UPS, which would convert (rectify) the AC output into DC, then use that DC power to generate a new AC waveform, via an inverter.

  21. Hello,

    I’m interested in purchasing generators for my business. I would be using them to run dehumidification equipment in areas where a storm (usually a hurricane) has knocked out power in the area.

    I’ve seen a model I like, but it has a THD of 25%, which FAR exceeds your recommendation. What effects would that have on my equipment? Does that high of a THD affect the power output of the generator itself?

    Also, would having the generator outside and the equipment inside the house mitigate the issues with THD?

    Thank you for ay help you can provide, and thank you for a very informative article! (even though my head hurts a little, lol)

    • Hi Thomas,
      THD doesn’t have a real impact on the amount of power a unit can output. It essentially affects the waveform.
      A high THD can result in higher temperatures in your equipment conductors, which will either greatly shorten the life of electronics components, or simply damage them.
      Having your generator outside and the equipment inside would not change anything. Your generator should be outside anyways. NEVER run your generator indoors. Make sure to read our generator safety tips.

  22. Hi Paul,

    Thanks for your great article, and making a complicated situation simple.
    I am wondering if you can provide me with advice on a scenario i am facing currently in which I think this would have an impact on?

    Thank you

  23. Thanks for the great article.

    What I can’t seem to get answered is if there’s a way to take a portable generator like the Champion Power Equipment Model #100297 and clean up it’s THD with something like an inline automated voltage regulator.

    Or maybe I don’t need it since I’m also using a Cyberpower LX1500GU UPS.

    Champion portable generators will output an industry standard total harmonic distortion (THD) rating of about 12%-20% depending on load applied.

  24. Hi Paul,
    Thank you for your response. I’m in the sailboat business and have used inverters for years, and have always tried to find a pure sine wave because I was told computers were happier with them. Does it make any sense to power the furnace with an inverter?

    • Hi again Bob, it depends on the furnace. In your case, you may indeed want a clean sine wave. To be honest, I do think that inverter generators are pretty much always the better choice. Your furnace may also highly require proper grounding of the generator. You should check with its manufacturer and/or with a licensed electrician if you’re unsure.

  25. Hi Paul,
    I wanted to thank you for a perfect description on a very complex concept of how electricity works. I had a power outage today and was excited to test my new Westinghouse generator in conjunction with a new Ruud 98 furnace and it’s Econet thermostat. With a winter storm hours away I thought great money well spent $11,000.00. Not so the furnace and or the thermostat were not loving the generator. With the help of your article I’m now looking the the bonded natural or the 23% harmonic distortion. Kind of feeling like I bought the wrong generator, time to look at inverter generators.
    Bob Reed

    • Hi Bob, thanks for the kind words! It’s probably the Econet thermostat that is full of electronics. Inverter generators will be somewhat limited in the total amount of power they can generate. However, you can find powerful “classic” portable generators that will produce a modified sine wave with a low THD (<5%).

      • Paul, I like to think of THD as bad power going for the ride but still needs to be burned up. A 10k watt generator with 25% THD means there is only 8k watts at 60 hertz and 2k watts at 120, 180, 240 etc hertz. Your incandescent bulbs and hot plate can use the 2k watts for light and heat. However the motors in refrigerator, freezer, furnace, AC etc can only use the 60 hertz portion and the other bad power from the other frequencies must be burned up in windings of these motors. You might not see any problems until overheating or worse happens. The other electronics are a question. Most of these convert AC to DC and I don’t think these are susceptible to high THD. You are right that modern residential appliances create THD. Examples include light dimmers, fluorescent light, or variable speed motors in high efficency furnaces and AC units will create THD.

        • I put a 500 wat isolation transformer Between the plug, my TV and router. In your opinion will that work?

        • A isolation transformer is not designed to remove the harmonics of 60 hertz sine wave. The distorted wave with 25% THD will look different after a isolation transformer but most of the THD remains. You would need a filter to remove the harmonics of 60 hertz -180 hertz and higher. It is best not to generate the harmonics in the first place.

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