well when inverter or ups manufactures say about backup time it is under ideal conditions of 27° C with a resistive load.where as ur 2 fans ,pc are all a inductive load so the actual power consumption of those devices may vary unlike a resistive load.
their will also be losses when the electrons move throught the wiring in ur house .
no idea about microtek their ups weren't so great.regarding the battery the amaron batteries are nothing more then rebadged automotive batteries,their weight is identical to their automotive batteries,wheres batteries from exide are such ELF range are true inverters batteries.
if the said microtech inverter operates on 12v then u will need a 160ah battery to get 5hrs of backup with 2 fans and a pc.
if the said ups runs on 24v system u will need a pair of 80ah batteries.