An online UPS runs your equipment from the battery circuit all the time, so there is zero gap in a power cut and the power stays clean. An offline UPS only switches to battery when the mains fails. Put servers and network racks on an online UPS; an offline or line-interactive one is fine for single desktops.
The three types of UPS
| Offline (standby) | Line-interactive | Online (double conversion) | |
|---|---|---|---|
| How it works | Passes mains through, switches to battery on a cut | Like offline, but also evens out small voltage swings | Always runs equipment through its inverter |
| Gap in a power cut | Very short switchover | Very short switchover | None |
| Power quality | Same as mains | Better than mains | Clean and steady |
| Cost | Lowest | Middle | Highest |
| Good for | Single desktops | Desktops, small network cabinets | Servers, storage, network racks |
Why servers need an online UPS
Servers, storage and the main network rack are the equipment that hurts most when it drops. With an online UPS, the equipment never sees the mains directly. When power cuts, nothing switches, so nothing restarts. It also filters out the voltage dips and spikes that slowly wear out power supplies.
The trade-off is cost, plus some extra heat and fan noise, because the UPS is working all the time. That is why it usually lives in the server room, not under every desk.
UPS or inverter?
This is one of the most common questions in Indian offices. An inverter is built to run lights and fans for hours on large batteries. A UPS is built to keep sensitive electronics running without a blink.
- Computers, servers, switches, firewalls, CCTV recorders: UPS. Most inverters switch over too slowly for these, and a restart can corrupt data or drop recording.
- Lights, fans, general sockets: inverter or generator.
Many offices use both: a UPS for IT and the CCTV recorder, and an inverter or generator for the rest of the building.
How to size a UPS
- List everything that must stay on: servers, storage, switches, firewall, CCTV recorder.
- Add up their power draw, then add headroom for growth.
- Decide how long it must run: just long enough to shut down safely, or long enough to bridge to a generator.
- Pick battery capacity for that runtime, and plan to replace batteries on schedule.
Batteries wear out even when they are hardly used. A UPS that has never been tested with a real power cut is a guess, so test it during planned maintenance. That is one of the checks in an AMC plan.
Last updated 4 November 2026