UPS (uninterruptible power supply)
Beginner
A big set of batteries that keeps the computers running if the power flickers or fails, until backup generators start.
Novice
Equipment between the grid and the IT load that rides through power disturbances using batteries or a flywheel. The common “double-conversion” type turns AC into DC and back to AC all the time, which costs a few percent of the power passing through it.
Expert
Double-conversion UPS efficiency rose from 85–90% in the 1990s to 95% or more; “eco” modes that bypass the inverter reach about 99% at some cost in conditioning. Efficiency falls at low load factor, which redundant (N+1, 2N) designs create. Some AI racks move short-term backup into battery backup units in the rack.