UPS's have 2 main purposes in the home or small business
Prevent a hard shutdown of the PC during a power outage, preventing data loss.
Have enough battery run time to allow a graceful shutdown of the PC.
Depending on the quality and model of UPS, they can also prevent low voltage or surge protection. More expensive small business UPS's can also perform Power Conditioning, actually improve the quality of the AC voltage supplied to the PC.
Choosing the correct size is somewhat difficult, because it is moderately difficult to determine just how much wattage your PC actually uses, this may also include a monitor if it is connected to the UPS. A 650 watt quality UPS should do the intended job (see #1 #2)
A quality UPS will include software and a data cable, with the software installed in the Operating System and the data cable connected from the UPS to the pc, the software can be configured to perform a graceful shutdown after a specified period of time (usually set mine to a 3 minute time out), in case you are not present when the power fails.
Home small business UPS's were never intended to give you a long run time while on battery, those types of UPS's are mainly for corporate data center use.
Spend your money on higher quality UPS rather than buying more UPS wattage than you need