FAQs - Standby UPS

Standby/Off-line UPS information including formats, battery runtime calculation, features and power protection levels.

A standby UPS is a Voltage and Frequency Dependent (VFD) uninterruptible power supply as defined by BS EN 62040. The output from the UPS is dependent upon the input voltage and frequency. There is usually no AVR/AVS to step the mains power supply voltage up or down and little spike/electrical noise filtering. The inverter if off-line (standby mode) and switches on to protect the load with battery power when the mains power supply fails.

Standby or off-line UPS do not have a built-in automatic voltage stabilise (AVS) to buck or boost the incoming mains power supply. The output of the off-line UPS tracks the input inside a voltage window. When the mains power supply fails outside this range the UPS inverter is switched on and supplies power to the connected load(s) from the small battery set.

This is dependent upon the size in Ampere-hours (Ah) of the battery set installed in the uninterruptible power supply. Most standby UPS have a small battery and their inverter sections are only designed to provide up to 5minutes runtime at their full rated load. Longer runtimes are made possible by over-sizing the UPS device i.e. using a 1kVA in place of a 500VA which can double the runtime supply available from the UPS battery.

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