Online and offline are different products

An online (double-conversion) UPS runs the load from its inverter continuously, so the load never sees a mains disturbance. An offline unit switches to inverter when the mains fails, leaving a transfer gap. For servers, medical equipment and industrial controls the difference is decisive.

Sizing

Add the real power draw of everything that must stay running, in watts.

Add the power factor of the load, since UPS ratings are usually given in VA.

Allow headroom for inrush when loads start.

Decide the redundancy: a single unit, or a parallel or modular configuration.

Confirm the supply and the load are both single-phase, or both three-phase.

Backup time

Autonomy is set by the battery, not the UPS. Required minutes, load in watts and battery capacity together determine the block. Capacity falls as load and temperature rise, and falls as the battery ages, so the figure at installation is not the figure in year five.

Where it goes wrong

Sizing from the nameplate sum instead of measured load.

Specifying autonomy without accounting for battery ageing.

Installing in an uncooled room, which shortens battery life dramatically.

Ignoring generator interaction — some UPS designs interact badly with a generator supply.

Getting it right

We size from the measured load and the required autonomy, then verify at commissioning with a load test.