Nameplate power ratings on gear are typically a worst-case maximum. If all PoE devices are at max and the fans are at max, high CPU, etc. you might come close to the 500 watts. A clamp-on ammeter with a sense loop (multiply amps times 120 for US) or inline wattmeter such as a Kill-A-Watt will tell you the consumption right now but not the maximum under different conditions.
Nameplate ratings on UPSes are often an optimistic marketer's best hope.
Get a UPS that is capable of 500 watts for safety, your switch should never pull more than that. Note that run time on the UPS is a function of battery capacity and load, not maximum rating.
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