Choosing a UPS: Offline, Line-Interactive or Online

An uninterruptible power supply keeps your equipment running through mains disturbances and short outages, and hands over cleanly to a safe shutdown when a longer one strikes. The right choice comes down to topology, and the three common types differ in how they treat the mains before it reaches your load.

The three topologies

• Offline / standby — runs your kit straight from the mains and switches to battery only when it fails. Transfer takes a few milliseconds. Cheapest and smallest; ideal for a single desktop PC or point-of-sale till.
• Line-interactive — adds automatic voltage regulation (AVR) to trim everyday sags and surges without touching the battery. Still a quick transfer on outage. The sweet spot for network cabinets, NAS and small servers.
• Online double-conversion — continuously rebuilds the waveform, so the load runs from the inverter at all times with effectively zero transfer time and complete isolation from mains noise. The choice for critical or continuous loads — comms rooms, medical and industrial control.

Sizing and runtime

Add up the wattage of everything you need to protect and allow headroom — a UPS is rated in both VA and watts, so check the load stays inside both. Runtime falls sharply as load rises, so if you need minutes rather than seconds to shut down (or to ride through a longer cut), size up or add external battery packs. Batteries are consumables; plan to replace them every few years.

Browse our UPS range, or tell us your load and we will help you match the right topology and runtime.