What “online” means here

A double-conversion UPS rectifies the incoming supply to DC, then inverts it back to AC. The load runs from the inverter continuously, so there is no transfer time when the mains fails. That is what distinguishes it from line-interactive and offline designs.

What this class of UPS suits

Servers and network cabinets

Small laboratory and instrumentation loads

Point-of-sale and security systems

Any single-phase load where the load must not see the mains disturbance at all

Points to establish before specifying

The connected load in watts, not the sum of the nameplate ratings

The backup time required, which determines the battery block

Whether the load is single-phase or needs a three-phase supply

Whether the site has generator backup, and how the UPS interacts with it

The ambient temperature of the UPS room, since battery life falls sharply with heat

What to verify on a supplied unit

That the output waveform and transfer behaviour match what the load needs

That the battery block supplied gives the autonomy specified at the actual load

That monitoring and shutdown signalling are configured

That the installation is commissioned and the load tested