The three most common types of Uninterruptible Power Supplies are:
- online double-conversion,
- line-interactive, and
- offline/standby.
In an online double-conversion UPS, the batteries are always connected to the inverter, so no power transfer switches are necessary.
Double-conversion UPSs are the safest, but most expensive kind of power protection.
The line-interactive UPS is similar in operation to a standby UPS, but with the addition of a variable voltage autotransformer.
The offline/standby UPS (SPS) offers only the most basic features, providing surge protection and battery backup.