You can extend the operating time of a typical UPS on battery mode by replacing the original small 12volt battery with a large car or deep cycle battery. Downside is recharging a depleted battery will take a long time but if the power outage is not often (not daily), the original charging circuit should be enough.
A few years back, I had a modified UPS capable of running over six hours. My brother had a larger battery and can run his PC for eight hours.





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