if frequency drops to 55 Hz, or the incoming voltage goes high to 140 vac, or drops to 90 vac, the inverter doesn’t care or even notice. It uses what it gets from the line and converts it to dc. It ALWAYS supplies noise-free 60 Hz/120 vac to its load.
I just wanted to point out that if frequency goes to 55, you have no power. The grid crashes a lot sooner than that. (For once, I know what I’m talking about. I’m from Texas!) And since I’m here, my PS Audio Power Plant 3, its entry-level regenerator, makes my system more articulate. It’s clearer, whatever that really means. Not a lot, but I kept it.