EMZETT.Power
In short: The electrical power supply of a computer system — from the wall outlet through the power supply unit to the regulated voltage for individual components.
In more detail: A PC power supply converts mains voltage (230 V AC in Germany) into several regulated DC voltages (e.g. 12 V, 5 V, 3.3 V) for the motherboard, CPU, drives and expansion cards. With USB devices, the USB port itself often handles power supply, without a separate power adapter.
In Depth
A PC power supply unit (PSU) handles several tasks at once: it converts the AC voltage common in households (230 V in Germany) into several stable, smoothed DC voltages that the individual components actually need — usually 12 V (for the CPU, graphics card, drive motors), 5 V and 3.3 V (for logic circuits on the motherboard). This conversion also has to remain stable under heavily fluctuating load (e.g. when a graphics card briefly draws considerably more power under full load), without the voltage dipping or overshooting — unstable power supply can lead to system crashes or, long-term, to hardware damage.
A power supply’s rated output (in watts) has to offer enough headroom for all installed components under full load, especially for graphics cards, which can briefly draw considerably more power under demanding tasks than their average value. With USB devices, power supply works differently: instead of their own power adapter, they draw the needed energy directly from the USB connection itself, where modern standards like USB Power Delivery allow considerably higher power levels (up to 240 watts) than classic USB — enough to even charge laptops over the same port that also transfers data.
Efficiency classes (80 PLUS)
Power supplies never convert electricity with 100% efficiency — some of the energy drawn is inevitably lost as heat. The “80 PLUS” certification system (Bronze, Silver, Gold, Platinum, Titanium, rising in efficiency) states what percentage of the energy drawn from the wall outlet actually arrives as usable DC voltage — an 80 PLUS Gold power supply wastes noticeably less energy than an uncertified one, which affects both the electricity bill and the waste heat generated inside the case.
Uninterruptible power supply (UPS)
For servers or critical systems, where a sudden power outage could cause data loss, a UPS (uninterruptible power supply, a battery with its own electronics between the wall outlet and the device) bridges a short period of time, to shut the system down in a controlled way or until a backup generator takes over — a sudden, unplanned voltage drop can otherwise damage not just ongoing write operations but, in the worst case, even the hardware itself.
See also: Power Distribution, Server