Another thing to consider is the time ratings are only good for a new, fully-charged battery. Battery capacity declines over time. I haven’t been involved in this field for a while, and batteries may have been improved, but when I administered a large office-building network, I considered, as a rule of thumb, 1/3 of the original battery capacity to be lost per year. Not only did I replace all batteries after 3 or 4 years, I took the decline into account for sizing the original unit purchase. For example, if I needed a 500VA unit to hold equipment up for 10 minutes, I would purchase a 1500VA unit, anticipating the third year of battery life.
Sure, I’ve had gel-cels last much longer than 3 years, but reliability was more important to me than squeezing out a few dollars of savings by taking a greater risk.
There’s nothing worse than expecting the unit to perform only to find out it is seriously underrated when something happens. And don’t forget that multiple power outages a few minutes apart won’t give the charging circuitry enough time to restore the full capacity, either.