Here is a block diagram of an UPS. Some considerations is the length of time you want the UPS to power the computer when the power goes off.
The 12 volt power supply needs to supply a little more current than than
the inverter requires to keep the battery charged. The power supply needs to be adjusted to about 13.6 volts to keep the battery charged.
When the line power goes off or drops low the computer will run on the battery. The battery needs to be selected to run the computer for the specified time.