Andy wrote on 3/31/2020 9:31 PM:
I would like to calculate the approx runtime for my UPS.

I did a search, but found conflicting formulas.

12 volt
18 Ah

Total use of computer, monitor, and router is about
200 watts.

The simplest and most efficient way is to plug two lamps with 100 watt
bulbs into your UPS and unplug everything else. Next unplug the UPS and
note the time. When the bulbs start to fade, the up time is calculated
by again noting the time and subtracting.

The use of engineering formulas isn't very accurate. There are a bunch
of highly nonlinear factors at play especially near situations were the
battery is almost used up. I think that even UPS manufactures supplement
theory with pull-the-plug-under-known load tests. Of course there loads
are more accurately known than the light bulbs I suggested but you'll
get a reasonable estimate fairly quickly.
--
Jeff Barnett