First part:. Different UPSs have different behavior. Some are more sensitive to disturbances, whether caused by line or load.

Second part, the UPS being connected to utility has no effect on the inrush to the server. It all depends on where in the AC sine wave the energy starts flowing. Ever plug something into an extension cord with a clear receptacle? Sometimes there is a sizable blue arc, sometimes small, sometimes none. All about where in the cycle the connection is made.

The size of your power supply matters. My server only draws 200w under load yet has a 1400w power supply. My 700va, 450w UPS would trip on start up as the inrush for the server was too large. I upgraded to a 1500va, 1150w UPS and it’s been fine ever since. Both UPSs were Eaton 5SLCD models.

I have one of these. 2 HDDs in the trays and SSD zip tied to the top of the cage. I also cut a hole in the side panel and added a fan as there is terrible air flow and my drives were getting quite hot.