Types
- Offline: switches to battery when the power goes out. Cheap, good enough for a computer.
- Line-interactive: also smooths out voltage fluctuations. Ideal for a NAS and network gear.
- Online: devices are always powered from the battery through an inverter. For servers and sensitive equipment.
Automatic shutdown
A UPS connected over USB lets the server know it is running on battery. After a while the NUT program then shuts down the NAS and other computers on the network so no data is damaged. Thanks to this, the RAID does not have to run a lengthy disk check after the outage.
Runtime
A UPS is not meant to run things for hours. It only needs to bridge short outages and, during a long one, shut devices down safely.