N — The Baseline
"N" represents the capacity actually required to support the load — for example, if a facility needs 3 UPS modules to carry its IT load, N = 3.
N+1
N+1 adds one additional unit beyond N, so a single module or unit failure does not interrupt power to the load — using the example above, N+1 would mean 4 UPS modules installed to support a 3-module load.
2N and 2N+1
2N duplicates the entire power path — two fully independent systems, each sized for the full load — for facilities that cannot tolerate any single point of failure. 2N+1 adds a further module of redundancy on top of that.