Online vs Line-Interactive vs Standby UPS: Which Do You Need?
The three UPS topologies explained side by side — what each one protects against, where it belongs, and why "cheaper" often costs more after the first outage.
The single most common specification error is buying a UPS that is the wrong topology for the load. A standby UPS that protects a printer is not the same device as the one protecting a server rack — even if both are labelled "UPS".
Standby (offline)
Mains flows straight to the load. The battery only kicks in when the input voltage drops below or rises above a set window, typically with a 4–10 ms transfer. Use it for non-critical edge devices, desktop PCs and small networking gear where a brief transfer gap is acceptable.
Line-interactive
Adds a transformer or buck-boost AVR so the UPS can correct moderate under- and over-voltage without switching to battery. This extends battery life in areas with a "noisy" supply. It still has a transfer break, so it is not suitable for loads that cannot tolerate any interruption.
Online double-conversion
The rectifier converts AC mains to DC, charges the battery, and feeds the inverter, which regenerates a clean AC sine wave to the load. There is no transfer break — the battery is always in the circuit. The load is fully isolated from mains disturbances, which is why this is the required topology for data centres, medical imaging, industrial control and any load where downtime is measured in thousands of pounds per minute.
Rule of thumb: if losing the load for 10 milliseconds would cause a problem, you need an online UPS. If it would not, line-interactive may save you money.
Compare specific online models across Riello, Legrand and Delta on our products page, or let the UPS Finder narrow it down for your load.
Talk to an engineer about your power protection
We specify, supply and maintain Riello UPS systems across the UK. Tell us about your load and we will recommend the right system.
