PSU redundancy (N+1, N+N)
Beginner
Having more power supplies than you need, so the server keeps running when one breaks.
Novice
N is the number of power supplies needed to carry the load. N+1 adds one spare; N+N (or 2N) doubles them, often so each half can be fed from a separate power feed.
Expert
Usable capacity is . N+1 covers a single PSU failure; N+N covers loss of a whole feed. Running supplies near half load also keeps them near their efficiency peak, but capacity reserved for redundancy is paid for and idle.
Explained in Board and server (Systems).