What a UPS actually does
A UPS sits between the wall socket and your equipment. It contains a battery and electronics that take over instantly if the mains power fails, so connected devices keep running without a flicker. Most also filter out spikes, surges and brownouts, which protects equipment from the electrical disturbances that are common during storms or when heavy machinery starts nearby.
A UPS is not a generator. It provides minutes of runtime, not hours. Its job is to ride through short interruptions, which are the most frequent kind, and to give equipment enough time to shut down safely during longer ones. Sudden power loss can corrupt databases, damage file systems and occasionally kill hardware, especially on servers and storage devices that are writing data at the moment the power goes.
Which equipment needs one
Prioritise anything shared or holding data: servers, NAS devices, firewalls, routers, core network switches and the fibre or internet equipment. If these drop out, the whole office loses access to files, internet and phones, even if the desktops are on laptops with batteries. Keeping the network up during a brief cut also keeps cloud phones and Teams calls going on laptops and mobile devices.
Desktops used for critical or long-running work, reception PCs running booking systems, point of sale terminals and phone system equipment are good candidates too. Laptops already have their own batteries, so they usually only need power to their dock and the network. Never plug laser printers into a small UPS; their heating elements draw large bursts of power that can overload it.
Remember the equipment that delivers your internet. The fibre ONT or other connection box supplied by your provider needs power too, and if it drops out the router has nothing to connect to. Put it on the UPS with the router where you can, so a short outage doesn't take down calls and cloud access along with the lights.
Sizing a UPS
Sizing depends on two things: the total load of the connected equipment and the runtime you need. Add up the actual power draw of each device in watts, ideally measured or taken from the manufacturer's specifications, then choose a UPS with comfortable headroom above that figure. Running a UPS close to its maximum shortens both runtime and battery life.
Manufacturers publish runtime charts showing minutes of operation at different loads. Most small offices aim for enough runtime to cover short interruptions plus a clean shutdown, often in the range of ten to twenty minutes for core equipment. If you need longer, some models accept extra battery packs. Your supplier or IT provider can help match the model to the load.
Types of UPS
Standby UPS units are the simplest and cheapest, switching to battery when power fails. They suit individual desktops and small home office setups. Line-interactive units also correct minor voltage dips and surges without switching to battery, which makes them the common choice for small server cabinets and network equipment.
Online, or double-conversion, UPS units run equipment from the inverter constantly, giving the cleanest power and no switchover time. They cost more and are typically used for servers and sensitive equipment in larger setups. Rack-mount versions fit standard comms cabinets, while tower versions sit on the floor or a shelf.
Automatic shutdown and monitoring
A UPS protects data only if equipment shuts down before the battery runs out. Connect the UPS to the server or NAS by USB, or fit a network management card, and install the manufacturer's software so it triggers a clean shutdown when the battery reaches a set level. Test this, because a misconfigured shutdown is one of the most common reasons a UPS fails to prevent damage.
Network-connected UPS units can send alerts by email when power fails, when the battery needs replacement, or when the load is too high. Add them to whatever monitoring your IT provider uses. Many outages happen overnight or at weekends, and knowing that equipment shut down cleanly saves a lot of worry on Monday.
Batteries, testing and safety
UPS batteries wear out, typically after three to five years depending on temperature and use. Most units have a self-test that warns when the battery is weak; don't ignore the beeping. Record battery replacement dates in your asset register and replace them proactively, because a UPS with a dead battery offers no protection at all when the power goes.
Keep the UPS in a cool, ventilated spot, as heat shortens battery life significantly. Don't plug a UPS into another UPS or a power board, and don't overload it with extra devices over time. If you need a new power circuit for a server cabinet or a large UPS, that work must be done by a registered electrician.
In short
- A UPS covers short outages and allows clean shutdowns; it is not a generator.
- Protect servers, NAS, firewall, router and core switches first.
- Size for real load with headroom and the runtime you need.
- Set up and test automatic shutdown on servers and NAS.
- Replace UPS batteries every three to five years.