THD stands for total harmonic distortion, and it measures how “clean” the electricity coming out of your generator is. A perfect power source produces a smooth sine wave at 60 Hz (in North America). In reality, every generator adds some distortion to that wave in the form of extra frequencies called harmonics. THD expresses the total amount of that distortion as a percentage: the lower the number, the cleaner the power.
How THD Works
Your utility power and your generator both aim to produce electricity as a smooth, repeating wave at a fundamental frequency of 60 Hz. But mechanical variations in the engine, fluctuations in load, and the design of the alternator all introduce additional frequencies (120 Hz, 180 Hz, 240 Hz, and so on) that ride on top of that fundamental wave. These are harmonics, and they distort the shape of the sine wave your appliances expect to receive.
THD is calculated by comparing the combined strength of all those harmonic frequencies to the strength of the fundamental 60 Hz signal. If a generator has 5% THD, that means the total energy in the harmonic frequencies equals 5% of the energy in the main signal. A generator with 25% THD is pushing a much rougher, more distorted waveform into your devices.
What Counts as “Clean Power”
In generator marketing, you’ll often see the phrase “clean power” or “pure sine wave.” These terms generally mean the generator produces power with less than 6% THD. That threshold comes from Generac and other major manufacturers, and it aligns closely with the IEEE 519 standard, which sets a 5% voltage THD limit for power producers at common residential and commercial voltages.
Inverter generators typically produce under 5% THD, which is why they’re recommended for sensitive electronics. Standard open-frame portable generators can have significantly higher THD, sometimes 15% or more, depending on design and load conditions. The difference comes down to how the power is produced: inverter generators convert their output to DC and then reconstruct it electronically into a clean sine wave, while conventional generators rely on the raw output of a spinning alternator.
Why THD Matters for Your Equipment
Simple resistive loads like incandescent light bulbs and basic space heaters don’t care much about THD. They convert electricity to heat regardless of the waveform’s shape. The problems start with anything that has a circuit board, a motor, or a digital display.
Furnaces are a common example. Modern furnaces use integrated electronic control boards to manage ignition, fan speed, and safety shutoffs. These boards expect clean power, and furnaces supplied with voltages above 8% THD may malfunction or suffer damage to their circuitry. The same applies to many HVAC systems, which depend on precise electronic controls to cycle compressors and manage refrigerant flow.
Electric motors are also vulnerable over time. High harmonic content forces a motor to work against frequencies it wasn’t designed for, generating extra heat in the windings. That heat degrades the insulation around the copper wire inside the motor. One study modeling motors in the 2 to 200 horsepower range found that voltage distortion significantly accelerates thermal aging of motor insulation, estimating the total cost of motor life lost to harmonic pollution in the U.S. at $1 to $2 billion per year. For a single generator powering your well pump or refrigerator during an outage, the effect of a few hours is negligible. But if you’re running equipment on a generator regularly, high THD shortens motor lifespan.
Other signs of high THD include LED lights dimming or flickering (especially cheap LEDs), buzzing from audio equipment, and erratic behavior from digital clocks or microwave displays. If your generator struggles briefly when a large load kicks on, you may notice a momentary brownout in connected lights as the voltage and waveform temporarily worsen.
Which Devices Need Low THD
Not everything you own requires an inverter generator. Here’s a practical breakdown:
- Sensitive to THD: Computers, televisions, gaming consoles, furnace control boards, variable-speed HVAC systems, battery chargers for power tools, and any device with a microprocessor.
- Moderately sensitive: Refrigerators, washing machines, and sump pumps with electronic controls. These will generally run on higher-THD power but may experience shorter component life or occasional glitches.
- Not sensitive: Incandescent lights, resistive heaters, basic power tools with brushed motors, and kettles. These devices don’t care about waveform quality.
Inverter vs. Conventional Generators
Conventional portable generators spin an alternator directly coupled to a gasoline engine. The engine speed determines the output frequency, so the generator must run at a constant RPM (typically 3,600) regardless of how much power you’re drawing. Any fluctuation in engine speed distorts the waveform. These generators commonly produce THD in the range of 10% to 25%, though some better-built models stay lower.
Inverter generators take a fundamentally different approach. The alternator produces high-frequency AC, which is converted to DC and then electronically inverted back into a 60 Hz sine wave using microprocessor-controlled circuitry. This electronic reconstruction keeps THD below 3% to 5% in most models and allows the engine to throttle up and down based on load, which also makes them quieter and more fuel-efficient. The tradeoff is higher cost per watt and generally lower maximum output compared to conventional generators of similar size and price.
How to Measure THD
If you already own a generator and want to know its actual THD, you have a few options. The most accessible tool for homeowners is a power quality meter or a multimeter with THD measurement capability, which you can find for $50 to $200. You plug it into the generator’s outlet, apply a load, and read the THD percentage directly.
For more precise measurements, technicians use spectrum analyzers. These instruments measure the power at the fundamental frequency and at each individual harmonic, then calculate THD automatically. The key detail is that THD changes with load: a generator might show 3% THD at half load and 8% at full load, or vice versa. Always measure under realistic conditions, with the appliances you actually plan to run connected and operating.
Many generator manufacturers list THD specs at rated load in their product documentation. If you’re shopping for a new generator, look for this number. If it isn’t listed, that’s often a sign the manufacturer isn’t confident in the figure. Any generator advertising “clean power” or “safe for electronics” should have THD under 6%, and reputable inverter generators will specify under 3% or under 5%.
Choosing the Right THD for Your Needs
If you’re buying a generator strictly for emergency backup to run a refrigerator, sump pump, and some lights, a conventional generator with moderate THD (under 10%) will handle the job. If your furnace has an electronic control board, which virtually all modern furnaces do, you’ll want to stay under 8% THD to avoid control board issues, and under 6% for a comfortable safety margin.
If you’re powering a home office, charging laptops, running a television, or connecting any equipment with sensitive electronics, an inverter generator with under 5% THD is the practical choice. The price premium pays for itself by protecting equipment that costs far more to replace than the generator itself.