PSU (power supply unit)
Beginner
The box in a computer that changes wall electricity into the steady kind that electronics need.
Novice
An AC-to-DC converter for servers. Its efficiency (output ÷ input) depends on how heavily it is loaded; the 80 PLUS program certifies levels from basic 80% up to Titanium and Ruby.
Expert
Front ends use power-factor correction then an isolated DC/DC stage. 80 PLUS Titanium (230 V internal redundant) requires 90/94/96/91% at 10/20/50/100% load. Redundant PSUs share load and so often sit near 50% or below; modern AI racks centralize them in power shelves feeding a 48 V-class busbar.
Explained in Power and cooling (Systems).
See also: Power conversion chain, VRM (voltage regulator module).