What’s your budget? I’m a big fan of old Xeon servers.

You have to look at the datasheet. It says 6 minutes at full load and 18 minutes at half. Full data can’t be listed because it’s a non linear curve. The website gives you a calculator at the bottom.

https://www.apc.com/us/en/product/SMC1500-2UC/apc-smartups-c-line-interactive-1440va-rackmount-2u-120v-6x-nema-515r-outlets-smartconnect-port-usb-and-serial-communication-avrgraphic-lcd/

You pay a huge premium for rack mount. You could buy 2-3 regular UPS for the price of one rack mount. I bought a shelf for my rack and have regular UPSs on the shelf. Note that you can only chain 1 ups into another without problems. So it’s best to separate devices onto different UPSs or buy one that supports plugging external batteries.

Watts is peak usage not watt hours. You can have two 1500 watt UPSs but one could last far longer because it has extra batteries but the same 1500 watt inverter as another model.