There has been a thread recently about swapping out Batteries in UPSs to extend runtime (eg:swapping 7Ah ones with 14 Ah ones to double run time). Manufacturers tend to say don't consider this, get a UPS with a external battery packs & extend the run time that way. which is fine, but devices that can do this tend to be the much higher capacity ones (ie 750Va+).
I've got a scenario (similar to https://community.spiceworks.com/topic/2157910-ups-battery-selection-for-extreme-runtime-5-6-hours-f...but even more extreme) which I suspect will be much more common going forward, where I could do with a very long run time (ideally 24hours+) but a very low power draw - around 30W. (here in the UK the old POTS phone system is being retired so it is not (or won't be shortly) *possible* to order an old POTS line & all lines will be VOIP. Therefore...