I have a device that draws 60W and a UPS with a 12V battery rated at 7.5 Ah.
Ignoring the various losses, according to my calculations 12V times 7.5Ah means that the UPS can provide 90Watt hours of energy. This means that my 60W device can run for 90 minutes.
Are my calculations right?
Also I know that lead-acid batteries like those found in UPSes cannot be drained 100%. So what percentage of that 7.5Ah is actually usable?
What’s the rated run time of the UPS? No UPS can run indefinitely with no load - there’s a certain amount of battery power consumed just to have the device active, and it’s this overhead that your calculations have ignored.
The smallest UPS that APC makes is rated at 350 VA, and their size calculator says it will run for about 16 minutes delivering 50 watts, and 6 minutes at 100 watts. For your 60 watt load, I’d estimate ten minutes of power would be available with this unit.
The VA rating is irrelevant. It just shows the max power the particular UPS inverter can deliver and has nothing to do with duration.
For the duration what matters is battery Ah rating