Grid power is a smooth sine wave. A pure sine wave inverter reproduces it; a modified sine wave inverter approximates it with a blocky stepped waveform that is cheaper to generate. For a lightbulb or a resistive heater the difference is academic. For anything with a motor, a compressor or a switching power supply, it is not.
What modified sine actually does to equipment
- Motors run hotter and less efficiently. Fridges, freezers, pumps, fans and power tools all draw more current on modified sine to do the same work, and the extra heat shortens their life. This is the main reason to care.
- Audible buzzing. Transformers, fans and speakers hum on the stepped waveform.
- Some electronics simply refuse. Many CPAP machines with humidifiers, some medical equipment, laser printers and certain battery chargers either misbehave or will not start.
- Microwaves take longer and run hotter, delivering less cooking power than their rating suggests.
- Variable-speed and inverter-driven appliances — increasingly common in modern fridges and AC units — often will not run at all.
Where modified sine is still fine
| Load | Modified sine | Why |
|---|---|---|
| Incandescent lights, resistive heaters | Fine | Purely resistive, waveform irrelevant |
| Simple corded power tools | Usually fine | Universal motors tolerate it |
| Basic phone and laptop chargers | Usually fine | Though some run warm |
| Fridge, freezer, pump, compressor | Avoid | Motor heating and shortened life |
| CPAP, medical equipment | Avoid | May refuse to run; not worth the risk |
| Modern LED lighting | Often flickers | Driver circuitry dislikes the steps |
| Anything variable-speed | Avoid | Frequently will not start |
The honest recommendation
Buy pure sine wave. The price difference has collapsed — it is now typically 20-40% on the inverter, which on a system where the inverter is a minority of the cost is small — and the list of things you must not connect to modified sine keeps growing as appliances get more electronically controlled.
The specific case where modified sine still makes sense is a cheap, small, single-purpose setup: running work lights and a corded drill off a truck battery, for example. As soon as a fridge, a CPAP or anything with a circuit board is involved, the saving is not worth it.
Reading the spec sheet
Watch the marketing language. Some listings describe output as "sine wave" without the word pure, or as "simulated", "quasi" or "stepped" sine — all of which mean modified. If the datasheet does not clearly state pure sine wave, assume it is not.
A related figure worth checking is total harmonic distortion (THD). Genuine pure sine inverters state a THD below about 3%; higher figures indicate a waveform that is cleaner than modified but not truly clean.
This applies to power stations too
Nearly every current portable power station outputs pure sine wave, but budget units and older stock sometimes do not — and it is exactly the spec people skip when comparing capacity and price. If you intend to run a fridge or a CPAP from one, confirm it before buying, as we do throughout the CPAP power station guide.
For a freezer specifically, the waveform matters as much as the capacity — see running a freezer off-grid.