Calculate VA of device 100-240v
Figure 1. The power supply module.

The PSU is rated at 400 W at 48 V DC so your calculation of \$ I = \frac {P}{V} = \frac {400}{48} = 8.3 \ \text A \$ is correct.

The Technical Specifications page states


  Maximum transfer efficiency: 93%(220 V, 100% load, 25°C)


so I would work with 90% for some safety. This means that the input power (at maximum load) would be \$ \frac {400}{0.9} = 444 \ \text W \$.

Your UPS should be rated in Wh so you divide this by 444 W to get the run-time in hours.



I notice that the output is -48 V (minus 48 V) so this looks like a telecomms PSU as these used a positive ground. I would look around for a 48 V DC UPS as these were once commonly used for PABX backup power and consist of a fairly simple 4 x 12 V SLA battery pack, a charger and some way of switching between your mains-powered 48 V and the battery.



Regarding input current: the device is using a "universal input voltage" switched mode power supply. At 100 V AC input the current drawn will be \$ P = \frac {P}{V} = \frac {444}{100} = 4.4 \ \text A \$ and at 200 V AC input it will be 2.2 A.