|All depends how long this UPS has to last after the main AC has been cut (interrupted)??|
Calculating your power consumption it should be around 300W. that is without the display(s).
On below website explanation and suggestion on selecting a UPS. If you have a unstable AC power network, make sure your UPS comes with AVR (auto Voltage Regulator) build in. Also keep in mind to check/replace the batteries after 3-5 years of use.
ex. if you choose a 1000VA capacity UPS you could keep going for 2-3 hours until UPS cuts off.