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Mission critical applications for data centers, financial institutions, telecommunications, and governments typically have generators and uninterruptible power supplies (UPSs) providing backup power to keep their critical information technology loads online. Tier I and tier II topologies according to ANSI/TIA-942 are very simple and straightforward with no redundancy or single distribution path with redundant components. Tier III and tier IV configurations have increasing complexity with multiple distribution paths, concurrent maintainability, and multiple active paths. This paper will explore the high-level compatibility, sizing, and design considerations for generators and UPSs in tiers I, II, III, and IV topologies.