What Is a UPS and How Does It Work?

An uninterruptible power supply (UPS) sits between your mains electricity and your critical equipment, conditioning the incoming power and instantly taking over from its battery if the mains fails. The goal is simple: keep sensitive loads running long enough to ride through a brief outage, or to shut down gracefully during a longer one.

The three UPS topologies

Not every UPS does the same job. There are three main topologies, and choosing the wrong one is the most common mistake we see on site surveys.

• Standby (offline) UPS — passes mains straight through to the load and switches to battery only when the input falls outside tolerance. Fine for a single workstation or a network switch, not for anything truly critical.
• Line-interactive UPS — adds automatic voltage regulation (AVR), so it can correct sags and swells without draining the battery. A good fit for small server rooms and edge cabinets.
• Online double-conversion UPS — converts the incoming AC to DC and back to clean AC, so the load is always fed from a regenerated, perfect sine wave. This is the only topology that fully isolates the load from mains disturbances, and the one we specify for data centres, healthcare and industrial applications.

What a UPS actually protects against

A UPS is not just a battery in a box. It defends against the full family of power disturbances defined in EN 50160: outages, sags, surges, harmonics, frequency variation and transient spikes. The battery handles the complete loss of mains; the inverter and rectifier handle everything else, continuously.

If you are unsure which topology your site needs, our UPS Finder walks you through it in two minutes and returns a shortlist of suitable models.