To answer your question, Yes. Both the Modules and the Power Supply draw power from the UPS (Mains).
The Power Supplies draw in (typically in an AC supplied system) AC power, then switch/transform the voltage to the necessary voltage, rectify and regulate it to meet the requirments of the chassis. This power is available to the modules. It's a chain. The modules draw what they need. A single chassis, with a single module drawing - say 250 watts, would only draw from the mains about 300 or so watts. Just because you have a 1000 watt power supply does not mean that power supply will always draw 1000 watts from the mains.
A power supplies rating is the MAXIMUM amount of power it is rated to deliver. Exceed this rating and if there aren't protection circuits in the power supply, you will burn it up. Because of these circuits, and the inherent inefficency of a power supply (depending upon it's design, this inefficency could be upwards of 25% of the TOTAL rated power), the power supplies consumes power at well. So if you had a power supply that was 90% efficent (10% inefficent) and was rated at 1000 Watts, then theoretically, you could consume up to a maximum of 900 watts for your modules - including any PoE those modules are supplying.
If I were sizing a UPS for a switch/router, if it was a smaller switch or router (35/36/37/38 hundreds) I would size it to the specs on the rear. If it were a larger switch/router (45/65/75/76 hundreds) I would do as previously described (adding all the wattage drawn as shown on the cli command) and adding about 25% of the total wattage rating for the power supply. This, however, would limit your growth potential. So plan accordingly.
