Glossary: UPS and power protection terms

A short glossary of terms you meet when choosing and running an uninterruptible power supply.

UPS

Uninterruptible power supply. Keeps the load running during a mains failure on battery power and protects against surges and noise.

VA and watts

Apparent power (VA) and real power (W). Size a UPS by watts — the actual power the load draws.

Line-interactive UPS

Topology with automatic voltage regulation (AVR). Cheaper than online, good for PCs and workstations.

Online UPS (double conversion)

Continuously feeds the load through an inverter, delivering a clean sine wave with no break. Needed for servers and critical loads.

Bypass

A path that routes power around the inverter — for maintenance or overload so the load is not dropped.

Runtime

How long the UPS holds the load on batteries. Depends on battery capacity and the power drawn.

Efficiency

The share of power reaching the load. Higher means less loss and heat.

Sine wave

The shape of the output voltage. A pure sine wave is safe for active-PFC power supplies and sensitive equipment.

Cold start

Starting the UPS from batteries with no mains present.

Hot-swap batteries

Replacing batteries without shutting down the UPS or the load.