How long should a UPS battery be sized for?

In this scenario, the uninterruptible power supply is part of the critical power installation in order to provide a filtered, clean and stable electrical supply to the connected loads when mains power is present. When the mains power supply fails or fluctuates outside preset thresholds, the UPS uses its battery set to provide power via its inverter. At the same time the generator receives a signal via the automatic mains failure (AMF) panel to start. A standby generator will start-up and be up to full power within 1-2 minutes. Most UPS systems are installed with 10-30 minute battery sets to cover any delay in generator start-up and allow a short time for investigation and corrective actions. If the generator cannot start and the UPS exhausts its battery before the mains power supply returns, the connected loads are dropped. This is why monthly generator testing is recommended alongside regular maintenance for both the generator and uninterruptible power supply by manufacturer certified service engineers.