How to calculate UPS battery power consumption in overall usage

Hi all,
I have two 144W UPS batteries to keep my electronics going when power is interrupted in my off grid cabin. I am not sure how these consume energy in a steady state once they are fully charged? Do they stop drawing or simply reflect the output being used by the items that are hooked up to it (ie somewhat neutral)? In the case they use up wattage then is it better to put as many items on one rather than have two? thanks

Efficiency of the UPS can be calculated like you would for the inverter. Power into the UPS, and power out of the UPS. Killawatt style plug in would do this or you can get a dongle that allows you to use a clamp meter.

Ah...thanks!