What Is a UPS and How to Size One for Office Equipment
Basics4 minInnovialabs editorial team
In short
A UPS is a battery with electronics that keeps equipment running through outages and power fluctuations; size it by watts with headroom and by the runtime your router, NAS and server need.
What a UPS is and why an office needs one
A UPS (uninterruptible power supply) is a device with a battery that instantly takes over when mains power drops out, sags or spikes. Its job is not to keep the office running for hours without electricity, but to:
- prevent equipment from shutting down abruptly and losing data;
- give you time to shut down servers and a NAS cleanly;
- keep the router and connectivity alive through short outages;
- protect equipment from voltage surges.
The three UPS types
| Type | How it works | Where it fits |
|---|---|---|
| Offline (standby) | Passes mains power through normally and switches to battery on failure | Individual workstations, inexpensive peripherals |
| Line-interactive | Like offline, plus automatic voltage regulation (AVR) that corrects low and high voltage without going to battery | Routers, switches, NAS, small servers — the sensible choice for most offices |
| Online (double conversion) | Converts power continuously: the load always runs from the inverter, with no transfer | Servers and critical equipment, locations with poor power quality |
Higher classes protect better but cost more and run warmer and louder. For a typical office, line-interactive is a good balance; online units are for a server room or an unstable grid.
Also check the output waveform: a "pure sine wave" is safer for servers and power supplies with active PFC than a "simulated" (stepped) sine wave.
VA vs watts
A UPS lists two ratings:
- VA (volt-amperes) — apparent power;
- W (watts) — real power the UPS can actually deliver to the load.
Watts are always less than or equal to VA, and the ratio depends on the model and is listed in the specs. Size by watts: if your equipment needs more watts than the UPS provides, it will not cope, even if the VA figure looks large enough.
How to size a UPS step by step
- List the devices that must run on the UPS.
- Find each device's power draw in watts: from the power supply label, manufacturer specs or a plug-in power meter. Labels usually show the maximum; real consumption is lower.
- Add them up and leave headroom — running a UPS at its limit is not advisable.
- Decide the runtime you need: how many minutes it takes to shut down a server cleanly, or how long connectivity should last.
- Check runtime in the manufacturer's table. Runtime is not linear with load: at half load a UPS lasts noticeably more than twice as long as at full load. Manufacturers publish runtime charts or calculators — rely on those.
- If runtime is too short, pick a model that accepts external battery packs.
- Check the number and type of outlets: not every outlet is battery-backed; some offer surge protection only.
What should always be on a UPS
- Router, optical terminal (ONT) and switch — they draw little power, and without them the office loses internet and VoIP.
- NAS and file servers — a sudden power loss can corrupt data and the disk array.
- Servers and CCTV recorders.
- PoE access points — through a switch that is itself on the UPS.
What usually should not go on a UPS: laser printers and MFPs (high inrush current), heaters, kettles. They overload the unit quickly.
Set up automatic shutdown
Most UPS units connect to a server or NAS via USB or a network card. Using the vendor's software or the NAS's built-in settings, you can define: if running on battery longer than N minutes or when the charge is low, shut the system down cleanly. Without this, a UPS only delays an abrupt shutdown.
Maintenance
- Batteries age even if the UPS rarely runs on battery. Watch the indicators and the service life stated by the manufacturer.
- Run a self-test periodically.
- Keep the UPS away from heat sources: high temperatures shorten battery life.