Offline (standby) UPS
An offline UPS feeds mains directly to the load and only engages its inverter when the mains fails. There is a brief transfer gap (typically 2–10 ms), which is fine for a desktop PC but unacceptable for servers or sensitive electronics. It is the cheapest option and best suited to home offices and non-critical loads.
Line-interactive UPS
Line-interactive adds an automatic voltage regulator (AVR) or transformer that corrects minor under- and over-voltage events without switching to battery. This preserves battery life and improves reliability for office IT, small server rooms and network closets. Transfer to battery still involves a short gap, but only for complete outages.
Online double-conversion UPS
An online UPS constantly converts incoming AC to DC and regenerates a clean AC output, so the load is electrically isolated from the mains at all times. There is zero transfer time to battery because the inverter is always running. This topology (classified VFI — Voltage and Frequency Independent) delivers the highest power quality and is the standard for data centres, healthcare, industrial control and any load that cannot tolerate even a momentary break.
All Riello Sentryum, Sentinel Dual, Multi Power and Legrand Trimod/Keor ranges we supply are online double-conversion UPS.
Efficiency modes within online UPS
Modern online UPS also offer high-efficiency operating modes to cut energy waste:
- ECO mode — runs on bypass when mains is stable, switching to inverter only on a fault (highest efficiency, slightly lower protection).
- Smart Active mode — automatically chooses between online and ECO based on mains quality.
- Energy Saving mode (modular UPS) — shuts down redundant modules at low load.
These modes can push efficiency from ~96% up to 99%, lowering cooling costs and TCO over the unit's life.
Frequently asked questions
Is an online UPS better than line-interactive?
Yes, for any critical load. Online double-conversion gives zero transfer time and perfect power quality. Line-interactive is a cost-effective choice for less sensitive office loads.
What does VFI mean?
VFI (Voltage and Frequency Independent) is the IEC classification for online double-conversion UPS, where the output is completely independent of the input voltage and frequency.
Does ECO mode reduce protection?
Slightly. In ECO mode the UPS runs on bypass and only transfers to inverter on a mains fault, so there is a tiny transfer gap. For most critical loads we recommend staying in true online (VFI) mode.
Which topology do data centres use?
Data centres use online double-conversion (VFI) UPS, often in modular form with N+1 redundancy, such as Riello Multi Power or Sentryum Rack.