Start with what you are protecting
Choosing a UPS begins with a clear picture of the load. List every device that must stay up during a power cut — servers, storage, networking, phones, security, building controls — and note its power rating in watts or VA and its startup (inrush) current. Only include genuinely critical equipment; printers, monitors and desk lighting are better served by surge protection or a separate, smaller UPS. This load profile is the foundation every other decision builds on.
Pick the right topology
For any business-critical load, online double-conversion is the standard — it regenerates a clean sine wave with zero transfer time and isolates the load from sags, surges and harmonics. Line-interactive suits small offices and network closets where minor voltage regulation is enough, while offline (standby) is only for non-critical desktop equipment. Read our full UPS technologies guide for the detail.
Size for power and headroom
Add up the real power (kW) of your critical load, then allow 20–25% headroom for growth and inrush. Remember that UPS are rated in kVA, and kVA only equals kW at unity power factor (pf 1) — modern Riello and Legrand units are unity pf, which simplifies the maths. Use our power & runtime calculator to convert amps or equipment lists into kVA, or the UPS Finder for an engineer-led recommendation.
Decide runtime, redundancy and form factor
Decide how long the load must stay up — 5–15 minutes for a graceful shutdown, longer if a generator is slow or absent — and add external battery cabinets or lithium-ion where needed. Then choose redundancy: N+1 for most server rooms and data centres, 2N for zero-tolerance sites (see redundancy explained). Finally, pick a form factor — tower for offices, rack for cabinets, or modular for scalable data centres.
Avoid the common mistakes
The five mistakes we see most: underestimating runtime, sizing in watts not VA (ignoring power factor), forgetting inrush on motors and blade servers, specifying a fixed system in a growing environment (choose modular instead), and skipping monitoring so faults stay hidden. A little planning up front avoids all of them — and our engineers will sanity-check your selection for free.
Frequently asked questions
How do I choose the right UPS?
List your critical load and its power, choose online double-conversion topology, size for kW plus 20–25% headroom, decide runtime and redundancy, then pick a form factor. Our UPS Finder walks you through each step.
Do I need an online UPS?
For any server, data centre, healthcare or industrial control load, yes — online double-conversion gives zero transfer time and clean power. Line-interactive is fine for small offices only.
How much runtime should I allow?
5–15 minutes covers a graceful shutdown or a generator start. For longer cover without a generator, add external battery cabinets or choose lithium-ion.
Should I choose a modular UPS?
If you expect growth or want N+1 redundancy with downtime-free maintenance, yes. Modular frames (Sentryum Rack, Multi Power, TriMOD) let you add capacity without replacing the frame.