UPS capacity, battery backup, Scalability, level of protection are some of the factors considered for its selection. Other parameters include Physical dimensions, location of site, Serviceability, efficiency of UPS.
Now a days, scalable UPSs are available along with traditional UPSs. The redundancy of the Datacenter can be N, N+1, 2N or 2(N+1). Batteries could be standard or intelligent, hot swappable.
UPSs can be connected in Parallel or completely isolated 2N redundant or in hot standby connection