Generally when I hear UPS I think of a small battery-powered uninterruptible power supply that is typically the size of a shoe box and is used to keep your computer going during a brownout, or for a half hour or so giving you time to shut down the cpu and save your work. Sounds like you're referring to something substantially bigger. What is it and how does it work? It maintains an entire apartment? Is that just the lights, or also the fridge? For how much time?
Ok I wish I could explain it more technically, which I will ask my husband tomorrow and explain further but the UPS for the apartment runs fan, lights and smaller appliances like charging a cell phone but no fridge or tv. However in Dominican I have used it for tv for an hour or so as well which is not advisable but it did work. If you keep the usage to the minimum like only the needed lights and one or two fans it usually lasts you about 8 to 14 hours if not more, I have seen two big batteries as one for primary use and second for backup and I have seen single battery making good use too ... then when electricity comes back it goes back on recharge mode. its quiet convenient and im surprised why no one knows about it or uses it here I have seen people use it in Asian countries too ...
I find it better then using Generator because generators use fuel also the fumes are awful ... the UPS doesn't have none of that and no noise too ... only after sometime you need to put in the battery water if I know correctly that is also i think takes about 5 to 6 months or until its needed