The actual UPS configuration required is chosen by run time and energy usage, and energy usage is dependant on the configuration of drives and shelves.
If you have a FAS2554 with 24 x 6TB internal drives, worst case power usage at 110V is 650W, with 560W being typical.
For a typical 15 minute minimum run time, supporting only this single shelf configuration, you would be looking for a 1500VA/1200W UPS that might only be rated for 5 minutes at full load, but 12 minutes or more at half load.
Try the UPS selector from APC, it should have NetApp models on their, you input the model and other details and it suggests the size of UPS based on capacity and runtime you need.
Thanks for the update!
From your provided specs, typical usage is 864W, but worst case is 1060W. For a 30 minute runtime, via APC's calculator, you would require a 2000VA UPS, which would use 74% of capacity.
Larger VA rated UPS will provide longer run time, allow larger loads, or both.
Remember to also size for network equipment, and cooling.