Is an online UPS worth the lower efficiency?

On a critical load, yes. The difference is typically 2 to 4 percentage points of the load, which on a 20 kW load is a few hundred euro a year. One unplanned stop of a production line or a data hall usually costs more than that in a single event.

Can a UPS replace a generator?

No. A UPS covers seconds to tens of minutes and exists to make the interruption invisible. A generator covers hours and needs 10 to 30 seconds to take load. Long outages need both, or a correctly sized storage system.

What autonomy should I specify?

With a generator on site, 5 to 10 minutes is normally enough to cover start and load acceptance plus a margin. Without a generator, the autonomy is a business decision: how long the process must survive, and whether a controlled shutdown is acceptable.

How often should the UPS batteries be tested?

Impedance or conductance trending every 6 to 12 months, capacity discharge testing at commissioning and then every 2 to 5 years, annually once the string passes 80 percent of expected life.

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UPS topologies compared

5 min readLast reviewed: September 2026

Short answer

Online double conversion UPS rebuilds the output waveform continuously, so the load never sees a transfer and never sees grid disturbance. Line interactive UPS regulates voltage and transfers to inverter in 2 to 10 milliseconds. Offline UPS only switches on when the mains fails, in 5 to 20 milliseconds. Critical IT, medical and process loads belong on online units. Offline units belong on single workstations.

Three architectures, sold under the same three letters, protecting very different things. This is what separates them, and where each one is the correct engineering answer.

The comparison table

Efficiency figures are typical at 50 to 80 percent load. Online units with an economy mode reach line interactive efficiency, but economy mode also gives up part of the protection, so it is a site decision rather than a default.

UPS topologies side by side
Topology	Transfer time	Output regulation	Efficiency	Typical range	Correct for
Offline (standby)	5 to 20 ms	None, mains passed through	95 to 98 percent	0.4 to 2 kVA	Single PC, till, router
Line interactive	2 to 10 ms	Voltage regulated by autotransformer	95 to 98 percent	0.5 to 5 kVA	Small servers, network cabinets, CCTV
Online double conversion	Zero, no transfer	Voltage and frequency fully rebuilt	92 to 96 percent	1 kVA to several MVA	Data halls, medical, process, telecom core
Modular online	Zero, no transfer	As online, with redundant modules	94 to 97 percent	10 kVA to 1 MVA per frame	Growing loads, N plus 1 without a second frame

Why transfer time matters more than it sounds

A modern switched mode power supply rides through roughly 10 to 20 milliseconds of loss, so an offline UPS often works in practice. The problem is what it does not do. Between outages, the load sits directly on the mains and sees every sag, surge, harmonic and frequency excursion. On a weak feeder, that is the wear mechanism that shortens the life of the equipment behind it. An online unit removes the mains from the equation entirely, which is why it is the only sensible choice for a load whose failure has a cost.

Modular or monolithic

Modular online UPS lets capacity and redundancy grow inside one frame by adding power modules, which suits a load that will double within the asset life. Monolithic online UPS is usually cheaper per kVA at a fixed final load, and has fewer internal connections. Decide this before purchase, because converting later means replacing the UPS rather than extending it.

What to specify, whichever topology you choose

Real measured load in kW, not nameplate. Power factor, because the kVA rating alone hides 20 percent. Autonomy at the load you will actually run, at the temperature the battery room will actually reach in August. Growth over the asset life. Bypass and maintenance arrangements, so the UPS can be serviced without dropping the load. Battery monitoring, because the batteries fail long before the electronics do.

Frequently asked

Is an online UPS worth the lower efficiency?: On a critical load, yes. The difference is typically 2 to 4 percentage points of the load, which on a 20 kW load is a few hundred euro a year. One unplanned stop of a production line or a data hall usually costs more than that in a single event.
Can a UPS replace a generator?: No. A UPS covers seconds to tens of minutes and exists to make the interruption invisible. A generator covers hours and needs 10 to 30 seconds to take load. Long outages need both, or a correctly sized storage system.
What autonomy should I specify?: With a generator on site, 5 to 10 minutes is normally enough to cover start and load acceptance plus a margin. Without a generator, the autonomy is a business decision: how long the process must survive, and whether a controlled shutdown is acceptable.
How often should the UPS batteries be tested?: Impedance or conductance trending every 6 to 12 months, capacity discharge testing at commissioning and then every 2 to 5 years, annually once the string passes 80 percent of expected life.