Clean Power: THD and What Electronics Actually Tolerate
The spec sheet says THD under 3 percent on one machine and stays conspicuously silent on another, and the internet swears a cheap generator will murder everything with a circuit board. The truth is calmer and more interesting: most modern electronics are far tougher than the folklore claims, a few device categories genuinely care, and knowing which is which lets you match the machine to the load instead of buying fear. Here is the working landscape.
What THD Actually Measures
Perfect AC power is a smooth sine wave. Total harmonic distortion measures how much of the delivered energy has leaked into multiples of the fundamental frequency — the jags, flat spots, and ringing stacked on top of the ideal wave. Utility power typically arrives under 5 percent. Inverter generators, which synthesize their output electronically from a rectified DC stage, hold 3 percent or better at any load. Conventional generators couple the alternator directly to engine speed, and their waveform quality rides on winding design and load balance — 15 to 25 percent THD is common, worse when heavily or unevenly loaded.
Frequency stability travels with this story. An inverter unit holds 60 hertz electronically regardless of engine speed. A conventional machine's frequency is its engine speed, so every load step means a momentary dip and recovery, and a mistuned governor means chronic drift.
Who Actually Cares
| Load type | Sensitivity | Why |
|---|---|---|
| Laptops, phones, TVs, LED lighting | Low | Switch-mode supplies rectify immediately; waveform shape barely matters |
| Resistive loads: heaters, kettles, incandescents | None | Distortion is irrelevant to a resistor |
| Induction motors: fridges, pumps, fans | Low–moderate | Tolerate distortion; run slightly hotter on bad waveforms over long periods |
| Audio equipment | Moderate | Harmonics ride into the noise floor as hum and buzz |
| UPS units (standby, line-interactive) | High | Actively judge waveform and frequency; reject what they dislike |
| Some CPAP, medical, and lab gear | High | Makers specify clean power; follow the manual, not the forum |
| Devices with zero-cross timing (some clocks, controls) | High | Distorted crossings scramble timing logic |
The pattern: the horror stories cluster in the bottom rows, and the bottom rows are a small minority of what a household plugs in during an outage. The fridge, the freezer, the phone chargers, and the TV shrug at a 20 percent THD waveform. The UPS under the desk is the one that files a complaint.
The Overload Wrinkle
THD is not a fixed property of a machine — it degrades as the generator approaches its limits, and it spikes when a big motor start drags voltage down. A conventional unit loafing at 40 percent load makes meaningfully cleaner power than the same unit pinned at 95 with a well pump cycling on top. The cheapest waveform upgrade available is headroom: run the machine in the middle of its band and the distortion, the frequency dips, and the voltage sag all shrink together.
Protecting the Genuinely Sensitive
When something in the high-sensitivity rows must run on generator power, three tools solve it in ascending order of thoroughness. First, an inverter generator as the source — the clean-power problem simply disappears at the origin. Second, a line conditioner between generator and device, which filters harmonics and regulates voltage for gear like audio racks. Third, the nuclear option: a double-conversion online UPS, which rectifies incoming power to DC and rebuilds a perfect sine wave from scratch — the device never sees generator power at all, only the UPS's own synthesis. That last arrangement is how server closets and medical setups ride out generator power indefinitely.
The Buying Translation
If the mission is electronics-heavy — home office, entertainment, CPAP camping — buy inverter and never think about this topic again. If the mission is heavy iron — well pumps, compressors, shop tools — a conventional unit spends the same dollars on watts instead of waveform, and the iron does not care. The mixed household does what mixed households do: a conventional workhorse for the big loads, with the sensitive corner either fed by a small inverter unit or armored behind a double-conversion UPS. Match the power to the load and the folklore loses its last teeth.
Bench Gear
The instrument that replaces guessing. Watch voltage and frequency live while loads cycle and you know exactly how much margin your generator has left — and whether the waveform panic applies to you at all.
Rebuilds generator power into a laboratory sine wave and bridges transfer gaps. The definitive answer for the equipment that genuinely cannot tolerate distortion.
THD is a real number describing a real phenomenon that matters to a short list of devices. Know the list, buy headroom, and put the clean-power money exactly where the sensitivity actually lives.
Frequency Drift: The Quieter Spec
One more number worth reading on conventional machines: governed frequency accuracy. A healthy governor holds 60 hertz within a hertz or so at steady load, dipping briefly on load steps. A tired or misadjusted governor drifting to 57 or 63 sustained bothers more equipment than waveform distortion does — motor speeds shift, some clocks and controls misbehave, and UPS units reject the supply outright. The plug-in meter reveals it instantly, and on most engines the fix is a governor adjustment documented in the service manual, performed at no load and verified under load. Frequency is the spec nobody quotes and everybody experiences; check it once a season and after any carburetor or linkage work.
FAQ
The working consensus is under 6 percent, which is roughly what utility power delivers, and inverter generators typically produce 3 percent or less. Conventional open-frame generators commonly run 15 to 25 percent, which is where the caution zone for genuinely sensitive gear begins.
Almost certainly not. Modern switch-mode power supplies in laptops, TVs, phone chargers, and LED gear rectify incoming AC immediately and are remarkably tolerant of waveform distortion. The realistic risks concentrate in audio hum, some medical devices, certain UPS units refusing the power, and older motor timing circuits.
The UPS is judging the waveform, frequency stability, or voltage window and rejecting it — line-interactive and standby UPS units are notoriously picky about conventional generators. Solutions include an inverter generator, a UPS with adjustable sensitivity, or a double-conversion online UPS that rebuilds the waveform entirely.
No. Surge protectors clip brief voltage spikes; they do nothing to harmonic distortion or frequency wander. For waveform problems the fixes are an inverter generator, a line conditioner, or a double-conversion UPS between generator and load.