How does a standby UPS protect from sags and surges?

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.