Hi,
It would help if you post some clear photos. Judging from previous experience, having seen quite a few UPS design, it is not very difficult to guess which IC it may be depending on the configuration. My best bet would be SG3525 or SG3524. There are other possibilities as well, including 555 and flip-flop, etc, but SG3525 and SG3524 seem most probable.
However, if it is a 28-pin IC, chances are, it could be a microcontroller, 16F72 seems very popular for UPS and inverter circuits.
Hope this helps.
Tahmid.