Its unfortunate the manufacturers use the misleading term "modified sine wave". It was never a sine wave to begin with. The basic waveform is actually DC current
that cycles between negative and positive at 60 Hz. Additionally the corners of the wave are electronically lopped off to make it a little closer to a sine wave.
This is the actual shape from one modified square wave inverter
as seen on an oscilloscope. If you know electronics
, you know this is an absolute garbage wave form. I don't think one could honestly call it AC.
Explanation of the differences...
In my opinion a modified square wave is very dirty power and should be avoided if you have that option. The reason is that not all devices can run, or run at full power, when powered by a modified square wave. A modified square wave can also introduce noise
into audio devices.
I used to have such an inverter
onboard untill large true sinewave inverters became available. The old square wave inverter caused a few wall power converters to go up in smoke. And as you know, "Once you let the smoke out, there's no putting it back in to fix it."