Pure sine wave vs modified sine wave, what actually needs pure sine?

[ZellDincht]

I’m trying to understand the basics of inverter waveforms and I keep getting stuck on pure sine wave versus modified sine wave. People online make it sound like modified sine will instantly break everything. Others say it is totally fine for most loads. I’m confused because both sides sound confident.

A specific moment that got me thinking was during a short outage last week. I borrowed a small inverter and ran a fan and some lights and it felt satisfying to see everything working again. Then I noticed a faint buzzing sound from the fan and it made me wonder if that is the kind of thing people mean when they say modified sine can cause issues. Nothing failed, but the sound made me question what is happening to the motor.

For those who have used both, what devices actually care in real life. Which loads have you seen get noisy or hot on modified sine and which ones do not care at all?

 

[King_Troad]

That buzzing would make me second guess too. My rule of thumb is pure sine if it has a motor, compressor, or you want it to run quiet and cool. Modified sine for simple resistive stuff and emergency only. What size inverter are you looking at.

 

[Alan]

Modified sine is usually fine for simple stuff like resistive loads and basic power bricks, but if you want one easy rule, assume anything with a motor, transformer, or “sensitive electronics” label prefers pure sine. The buzzing fan you heard is a classic modified sine clue. It can run, but it may run hotter or less efficiently.