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ITGlossary · UPS technology
Offline (Standby) UPS
A UPS that leaves the load on the mains and switches to battery only when the mains fails, with a short transfer time; the simplest and cheapest topology.
Technical level:Basic
In normal operation the load is fed straight from the mains, usually through a simple surge filter, while the inverter stays on standby. When the voltage is lost or goes out of range, a relay moves the load to the battery within a few milliseconds.
This design suits single computers, routers and home equipment, where cost matters most. It cannot correct undervoltage or overvoltage without falling back on the battery, so on a grid with frequent fluctuations the battery wears out faster. Many models also deliver a stepped approximation of a sine wave when running on battery.
A common mistake is choosing one for servers or sensitive equipment; a line-interactive or online UPS is the safer choice there.
Related terms
- Transfer TimeThe brief power gap when a UPS switches from mains to battery, typically 2–10 ms for line-interactive models and zero for online ones.
- Transfer SwitchA manual or automatic switch that connects the load to one of two sources, such as the grid and a generator, never both at once.
- Transformerless UPSModern UPS design that omits bulky transformers to improve efficiency and reduce size.
- Line Interactive UPSA UPS that corrects voltage sags and swells with an automatic voltage regulator and switches to battery only when the voltage is lost or out of range.
- Rack-Mount UPSA UPS designed to fit a standard 19-inch rack, with its height measured in rack units (U).
- Static SwitchA thyristor-based switch with no moving parts that transfers the load between inverter and mains within a fraction of a cycle.