mm writes:

Unless it's a hybrid UPS with ferroresonant line conditioner, it's
standby / float charging power consumption is pretty negligible.

Then why is my one-computer UPS so hot if I leave it on while not
using the computer. Not just in one spot, but over the entire top
surface and some of the sides.

What make and model? There *are* true UPSes that convert line current
to DC and then back to AC continuously, running the inverter all the
time. The supposed advantage is that there's no discontinuity in the
output waveform when the input line voltage goes away, but these UPSes
are larger and heavier and dissipate significant power continuously.

Or you could have a unit with a ferroresonant voltage regulator, which
wastes about 20% of its full load output rating in heat continuously.

But most people use standby power supplies that switch on only when AC
fails. These dissipate little power when on standby (just enough to
charge the batteries and run the electronics). Voltage regulation can
be provided by tap-switching transformers, again with little heat
generated (some SPSes include this function).

I have a variety of APC SPS units, and none are more than slightly warm
to the touch when operating in standby.

Dave