What a UPS actually does
An uninterruptible power supply (UPS) is a device that sits between your mains supply and your critical equipment, providing clean, continuous power even when the mains fails, sags or surges. The moment the mains drops outside acceptable limits, the UPS switches to its battery inverter so seamlessly that the connected load never notices the interruption.
That near-instant transfer is the whole point: servers, switches, medical systems and industrial controllers can crash or corrupt data on a power break lasting just a few milliseconds, while a UPS bridges the gap until the mains returns or a generator takes over.
The three main types of UPS
UPS systems are grouped by how they process power:
- Offline (standby) — passes mains straight to the load and only switches to battery when the mains fails. Cheap, but with a tiny transfer gap.
- Line-interactive — adds automatic voltage regulation (AVR) to correct minor sags and surges without draining the battery. A good middle ground for offices and small server rooms.
- Online double-conversion — converts incoming AC to DC and back to AC continuously, so the load is always running on a regenerated, clean sine wave with zero transfer time. This is the standard for data centres, healthcare and any mission-critical load.
Most of the Riello and Legrand ranges we supply — Sentryum, Sentinel, Multi Power, Trimod and Keor — are online double-conversion UPS.
What a UPS protects against
A UPS does more than ride out a blackout. It also filters out the everyday disturbances that quietly damage equipment and corrupt data: voltage sags and swells, spikes and transients, frequency variation, harmonic distortion and brief dropouts. By conditioning the power as well as backing it up, a good UPS extends the life of the hardware it feeds.
Choosing the right UPS
Sizing a UPS means knowing your total load (in kW or kVA), the runtime you need on battery, the redundancy you want, and the environment the unit will live in. Our UPS Finder walks you through these questions and returns an engineer-led recommendation, or you can use the power & runtime calculator to estimate your requirements first.
Frequently asked questions
What does UPS stand for?
UPS stands for Uninterruptible Power Supply — a device that provides continuous, clean power to critical equipment during mains failures and disturbances.
How long can a UPS keep equipment running?
A typical UPS provides 5–30 minutes on internal batteries — enough for a graceful shutdown or for a generator to start. Runtime can be extended to hours by adding external battery cabinets.
Do I need a UPS if I have a generator?
Yes. A generator takes 10–30 seconds to start and stabilise, while a UPS bridges that gap with no break in power. They work together: the UPS holds the load, the generator sustains it.
What size UPS do I need?
Add up the power (kW) of everything you want to protect, add 20–25% headroom for growth and inrush, and choose a UPS rated above that figure. Unity power factor (pf 1) UPS make kVA equal kW, simplifying the maths.