The stabiliser goes in front of the UPS, not instead of it

A question that comes up on the counter most weeks, usually from a shop or a clinic on a supply that misbehaves: do I need a stabiliser or a UPS? The honest answer is that they fix different faults, and a site that has both faults needs both boxes.

A servo stabiliser corrects voltage continuously. When the supply sits at 190 volts all afternoon, or swings to 260 when the neighbour's load drops off, the stabiliser holds the output at 230 and does it with a motor-driven transformer, not a battery. It has no answer to an outage: when the supply goes, it goes.

A UPS carries the load through the outage, but it corrects voltage only within a narrow window. Outside that window it switches to battery, which is exactly what it does on a wandering supply: dozens of shallow discharges a day, a battery that is never fully charged when the real cut comes, and a set that fails its first real test after eighteen months. That is the most common UPS complaint this desk hears, and it is not a UPS fault.

The arrangement that works is the stabiliser first, on the incoming supply, and the UPS behind it, so the UPS only ever sees 230 volts and only ever switches to battery for an actual outage. On a good supply the stabiliser is unnecessary. The way to know which supply you have is to log it for a week, which a cheap meter does.