In addition to parallel-capacity and parallel-redundant, there are other configurations to consider:
Isolated-Redundant: this is sometimes known as N+1 but it differs considerably to a parallel-redundant N+1 installation. In this configuration, there’s a main UPS module that feeds the load, while the secondary UPS feeds its static bypass. When a fault causes the primary UPS to transfer to bypass, the secondary module accepts the full load.
Distributed-Redundant: also known as Tri-Redundant and typically used in large multi-megawatt data centres. Made up of three or more UPS with independent input and output feeders, with the output buses connected to the load by several PDUs (and in certain cases Static Transfer Switches). This configuration does minimise single points of failure and offers the opportunity for concurrent maintenance, but it also leads to significant load management challenges.