Total harmonic distortion (THD)
THD measures how far a generator’s AC output deviates from a pure sine wave, as a percentage. Under 5% THD is generally considered safe for sensitive electronics; conventional open-frame generators can exceed 15–25%, while inverter generators typically stay under 3%.
Distorted power stresses switch-mode power supplies, motor controls, and microprocessor-based appliances — modern furnaces, variable-speed pumps, and audio equipment are common casualties of dirty generator power.
Manufacturers of conventional generators rarely publish THD figures; “clean power” marketing without a number is not a specification. If electronics matter, an inverter generator (or a published sub-5% THD figure) is the safe choice.
| Definition | RMS of all harmonic components ÷ RMS of the fundamental, expressed as a percentage |
|---|---|
| Inverter generators | Commonly specified under 3% at rated load; some under 1.5% at no load |
| Conventional open-frame | Roughly 5–15% with a good AVR; capacitor-excited units can exceed 20% |
| Modified-sine-wave inverters | Roughly 25–45% — not the same thing as an inverter generator |
| Utility power at a household outlet | Typically 1.5–5% |
| IEEE 519-2014 voltage limits | 8% THD at 1 kV and below; 5% for 1–69 kV systems |
What the number is measuring
A perfect 60 Hz sine wave carries energy at 60 Hz and nowhere else. A real generator also puts energy at integer multiples — 180 Hz (the third harmonic), 300 Hz (the fifth), 420 Hz (the seventh). Total harmonic distortion is the RMS sum of all that harmonic energy expressed as a fraction of the fundamental. Odd harmonics dominate because the distortion in a rotating machine is symmetric about the zero crossing, and the third and fifth usually carry most of it, produced by winding distribution, stator slot geometry, and the shape of the rotor’s magnetic field.
THD is not the whole of power quality. Voltage regulation — how far the voltage sags when a load comes on, roughly ±5% on a decent automatic voltage regulator and ±10–20% on cheap capacitor excitation — matters at least as much. So does frequency stability: an inverter holds 60 Hz to a fraction of a hertz because a crystal oscillator sets it, while a conventional governor hunts by a hertz or three as load changes. A generator can post a modest THD figure and still be hard on equipment if the voltage swings 25 V every time the refrigerator starts.
And THD is load-dependent, which is the caveat that gets omitted. Published figures are usually measured into a linear resistive load, sometimes at no load at all, which is the flattering case. Feed the same generator a rack of switch-mode power supplies — non-linear loads that draw current in short peaks near the voltage crest — and both current and voltage distortion rise. A spec sheet that says “<3% THD” without stating the load condition is quoting a best case.
What distorted power actually does to equipment
Switch-mode power supplies — in laptop chargers, LED drivers, TVs, and appliance control boards — rectify incoming AC into a DC bus behind an electrolytic capacitor. Harmonics change the relationship between peak and RMS voltage and increase the ripple current that capacitor has to absorb. Electrolytics run hotter as a result, and their life roughly halves for every 10 °C of extra internal temperature. That is the mechanism behind chargers and control boards failing weeks after an outage rather than during one, which is also why the damage is so rarely attributed to the generator.
Motors and transformers see harmonic currents that do no useful work but produce eddy-current and hysteresis losses that scale with frequency, so the fifth and seventh harmonics heat iron out of all proportion to their amplitude. Variable-speed drives and ECM blower motors add a second failure path: many use zero-crossing detection for timing, and a waveform with flattened or noisy crossings can push a control board into a fault state. Furnace control boards are among the most commonly reported generator casualties for exactly this reason.
Plenty of things do not care at all. Incandescent lamps, resistance heaters, water-heater elements, and universal-motor power tools convert whatever waveform arrives into heat or torque and are indifferent to its shape. If your outage load is a well pump, some lamps, and a space heater, THD is close to irrelevant. If it is a condensing furnace with an ECM blower, a variable-speed pool pump, and a home office, it matters.
How to specify it, and how to verify it
The 5% threshold quoted everywhere is a convention rather than a rule. IEEE 519-2014 sets recommended voltage-distortion limits by system voltage: 8% THD at 1 kV and below, and 5% from 1 kV to 69 kV. The “under 5%” figure in generator marketing effectively borrows the medium-voltage row. Where a hard limit exists for you it will be in the manual of the device you are protecting — some CPAP, oxygen-concentrator, and medical-device manuals state “pure sine wave required” or give a maximum distortion figure, and that is the number to design to.
Verification is harder than it looks. A true-RMS multimeter reads voltage accurately but tells you nothing about harmonic content; you need a power-quality analyser or an oscilloscope to see the waveform. In practice buyers rely on the published figure, and the only category that reliably publishes one is inverter generators. Conventional manufacturers mostly publish nothing at all, and “clean power”, “digital AVR”, and “safe for sensitive electronics” without a percentage are not specifications.
You do not have to buy waveform quality from the generator. A double-conversion — online — UPS between the generator and the sensitive equipment rebuilds the waveform from its own DC bus and fixes THD, voltage sag, and frequency wander in one device, for whatever load you put behind it. For a router, a laptop, and a CPAP machine, a few hundred watts of online UPS is often a better answer than a $1,000 step up in generator price. Beware the opposite problem with cheaper line-interactive UPS units: many reject generator power outright because the frequency wanders outside their acceptance window, and sit on battery until it dies. Widening the sensitivity setting, if the model allows it, is the usual fix.
Frequently asked questions
What is a safe THD for electronics?
Under 5% is the common threshold and under 3% removes the question entirely. Inverter generators typically publish figures in the 1–3% range; conventional open-frame units run roughly 5–15% and sometimes over 20%. Where a specific device matters — a CPAP machine, an oxygen concentrator, a medical monitor — its own manual is the authority, and some of them specify pure sine wave explicitly.
Will a conventional generator damage my laptop or TV?
Not usually on the first outage, and rarely in a way you can see happen. The realistic risk is cumulative: harmonic content and voltage swings stress the electrolytic capacitors in switch-mode power supplies, shortening their life so the device fails months later. A well-regulated conventional generator under 10% THD is fine for most household electronics. The reason to specify an inverter is that it removes the judgement call.
How can I measure my generator’s THD?
You need a power-quality analyser or an oscilloscope with harmonic analysis — a multimeter, even a true-RMS one, cannot do it. Most owners never measure it and rely on the manufacturer’s published figure, which is why the absence of a published figure on conventional generators is itself informative.
Do all inverter generators have low THD?
In practice yes, because the waveform is synthesised electronically rather than produced by a spinning alternator. The ones worth buying publish a number and state the load condition it was measured at. Watch the naming: a modified-sine-wave inverter — the kind sold as a car power adapter — is a completely different device with 25–45% distortion, and shares only the word “inverter”.
Does a surge protector fix dirty generator power?
No. A surge protector clamps brief high-voltage transients — lightning, switching spikes — and does nothing at all about a continuously distorted waveform or a sagging voltage. What addresses waveform quality is a voltage regulator, a line conditioner, or a double-conversion UPS, in ascending order of effectiveness and cost.