A power cut of just a few seconds is enough to reboot the router, drop a video call and, in the worst cases, corrupt the data on a NAS or server caught in the middle of a write. An uninterruptible power supply (UPS) keeps equipment powered through voltage dips and, if the outage drags on, gives you time to shut it down cleanly. It is one of the least glamorous components of an infrastructure, and one of the most useful.
The three types
| Type | How it works | Pros | Cons |
|---|---|---|---|
| Standby (offline) | Normally passes mains power through; switches to battery when there is a problem | Cheap, efficient | Short transfer time, no voltage correction |
| Line-interactive | Like standby, but with a regulator that corrects sags and surges without using the battery | Good compromise, uses the battery less | Transfer still occurs |
| Online double conversion | Continuously converts power: equipment is always fed by the inverter | Always-clean output, no transfer | More expensive, noisier, less efficient |
For home and small office, a line-interactive unit is almost always the right choice. Online units are justified for sensitive equipment, very disturbed mains or specific professional needs.
Waveform
Cheap UPS units produce, when running on battery, an approximated (stepped) wave rather than a pure sine wave. Many modern computer and server power supplies with active power factor correction can misbehave with this waveform: buzzing, shutting down or failing to ride through the transfer. For NAS units, servers and quality PCs, a UPS with pure sine wave battery output is advisable.
Sizing: VA, watts and runtime
Two values appear on a UPS:
- VA (volt-amperes): apparent power;
- W (watts): the real power it can deliver, usually lower.
The figure that matters for sizing is the watt rating. The procedure:
- Add up the real consumption of the equipment to protect. A plug-in power meter is more reliable than nameplate values, which show the maximum.
- Choose a UPS with a watt rating at least 30-40% higher than what you measured, for headroom.
- Check the manufacturer's tables for runtime at the expected load: at low load it lasts much longer than at full load.
A concrete example: router, ONT, a switch and a small NAS might draw 40-60 W in total. A small line-interactive UPS keeps them running long enough to ride through short outages, and to shut down cleanly if the outage lasts.
What to connect (and what not to)
Connect to the UPS:
- ONT or modem, router and main switch: without them the network stops, and keeping them on means you stay connected through short outages;
- NAS units and servers, which are the most vulnerable to data corruption;
- possibly PoE access points, via the switch.
Do not connect laser printers, heaters, hair dryers or other loads with heavy inrush current: they can overload the UPS. Many models have outlets with surge protection only, no battery: use them for non-critical devices.
Automatic shutdown
If the outage outlasts the runtime, the UPS switches off and the equipment with it. To prevent NAS units and servers from being cut off abruptly, you need a shutdown signal:
- the UPS connects via USB to the NAS or server, which reads its status;
- when the battery drops below a threshold, or after a set time since the outage began, the system starts an orderly shutdown;
- with NUT (Network UPS Tools) or the built-in features of many NAS units, a device connected to the UPS can also notify others over the network, which then shut down in turn.
Also configure automatic power-on when mains returns in server BIOS settings and NAS settings, otherwise they will stay off until someone presses the button.
Batteries and maintenance
The lead-acid batteries used in most UPS units deteriorate over time, faster in warm environments. After a few years real runtime may be a fraction of the original, and the UPS may shut down exactly when it is needed.
Good habits:
- place the UPS somewhere ventilated, away from heat sources;
- run the battery test periodically from the panel or software;
- note the battery installation date and plan its replacement;
- watch for notifications: many UPS units signal when the battery needs replacing.
Some newer models use lithium iron phosphate batteries, with a longer service life, at a higher upfront cost.
In short
- A UPS protects against short outages and allows an orderly shutdown during long ones.
- For home and small office, a line-interactive unit with sine wave output is the most balanced choice.
- Size in watts with headroom and check runtime at the real load.
- Connect ONT, router, switch, NAS and servers, not heavy loads like laser printers.
- Configure automatic shutdown, and test and replace batteries regularly.
