What is the UPS of a computer? How does it work?

UPS is Power Backup for your PC, when your mains power goes off, PC takes power from The UPS and when mains comes back it switches back to Mains. UPS is very Similar to Your Home Inverter but provides superior quality of power for your PC than a normal inverter.

UPS comes in various ratings, A 600VA (330Watts) or 1KVA (660Watts) are common. When buying a UPS you shall check your PC’s SMPS capacity and Use a higher capacity UPS for your PC.

Basic UPS supports upto 330Watts hence if Your PC SMPS is 250/300 Watts - this one is Okay to buy. 550Watts SMPS shall use a 1KVA UPS which supports 630/660 Watts load.

These are the UPS pages over here http://bwindia.net/catalog/ups-related-products/ups-4-computers