N+1, also called parallel redundancy, is a safeguard to ensure that an uninterruptible power supply (UPS) system is always available. N+1 stands for the number of UPS modules that are required to handle an adequate supply of power for essential connected systems, plus one more.
If an enterprise data center uses a single large UPS, and that module fails, systems will be disrupted. In an N+1 scheme, multiple small UPS modules and batteries are integrated together. During normal operations, the load is shared equally across all modules, which behave as if they were a single large UPS device. If a single module fails or needs to be taken offline for service, the UPS system will still be able to provide an adequate supply of power because it's already been configured with one extra module.
When N+1 is used for redundancy, the UPS modules should be sized so that the total anticipated load can be carried by three modules. The benefits of N+1 diminish after that point.
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