hi dude.
well, ok so a 800VA ups wud do really gr8 for ur system, go in for a APC ups
u need not (and i think shud NOT) use the UPS along with an inverter, instead it wud be better if u connect the UPS to work directly from mains (the 5 Amp socket in ur room) and wen the electricity goes, the ups will provide steady power to ur PC.
well On line UPS means Supply will come from batteries on line
and charging also on line.
In OFF line UPS supply will come from batteries when main
line supply fails.Batteries will charge on line.
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in ur case, if the power fluctuation is really a big concern, then get a line interactive UPS (as it helps to protect power surges better than online UPS)..