UPS needs to be sized twice the size as needed to cover in rush or the UPS needs to specify it can handle in rush demand up to 2x its rating.
So for instance our 250W switch needs a UPS rated at 500W or the UPS needs to say it can handle in rush to 2X its rating then you could use a 250W UPS but keep in mind you need to add up all your devices and size to 2X that size.
It does not matter that you will not power 250 watts without switch the 250W power supply will use close to 500W in rush for a very short time during the inrush.
Personally most WISPs have stopped using drycell computer UPS systems for AGM Battery Inverter Chargers like the TrippLite units
I use the 1250W versions
https://www.walmart.com/ip/Tripp-Lite-P ... 3=&veh=sem