Selection starts with measurement

Log the incoming voltage across the worst period of the day and week. A regulator specified on nominal voltage will pass the worst condition straight through.

What follows from that

The correction range, the rating, the phase configuration and the required output accuracy. Where the site has motors, the starting current sets the rating rather than the running current.

Support matters for a mechanical product

A servo regulator has a motor, a control board and moving contact. Ask who serviced it locally, how fast a servo or board can be replaced, and whether spares are held. Those answers determine the real cost of ownership.