There are various types of lead acid batteries. Some are designed to handle brief, deep discharges at a high rate of current, such as automotive starting batteries. Others can handle low current discharges but can be more deeply discharged at frequent intervals, such as marine batteries designed to run electric trolling motors. Golf cart batteries are designed for frequent, deep, high current discharges (they are also heavier). Golf cart batteries also can handle floating on a trickle charge better than most other lead acid batteries.
One could make a super USP by using two or more golf cart type batteries (they are usually 6v so they need to paired in series to get 12v), a 120v to 12v converter to keep it charged, and a pure sine wave inverter to convert the 12v back to 120v. The inverter and converter are standard RV (caravan to you all on the other side of the pond from me) equipment. One just needs to be sure the inverter is pure sine wave instead of modified or stepped sine wave (many power supplies will work only on pure sine wave). The converter needs to protected by a good, RV type EMS (Electrical Management System) that provides surge and spike protection and will shut off the incoming 120v if it goes too low or too high (these are expensive, at least $350 US, not some $50 cheapie).
To use as a UPS, the batteries would be fed by the converter (it needs to be large enough to handle all of the load being connected to the "UPS"). The inverter would run off the batteries. The electronics being connected to the "UPS" would be plugged in to the inverter's 120v output. By running all the time on battery fed inverter power, the electronics will always have an uninterupted source of power, no matter how often or frequently the incoming 120v may be interrupted.
Depending on how big the batteries are and hw much load in put on the "UPS", a setup like this could run for hours. However, one will pay through the nose (and other body orifices) for such a setup. I plan on doing something like this when I move into a travel trailer since much of the equipment will be already there.