hardware hardware

PSU (PSU)

psu power-supply hardware efficiency pc-building
Plain English

The PSU is the computer’s power supply. Wall outlets deliver high-voltage alternating current, but computer parts need low, steady direct current. The power supply converts one to the other and feeds every component. A weak or failing power supply causes random crashes and reboots that look like other problems, so it matters more than its price suggests.

Technical Definition

A Power Supply Unit (PSU) converts AC mains (120 V or 230 V) to DC rails: +12 V (CPU, GPU, most load), +5 V and +3.3 V (logic, SATA drives), and +5 V standby (wake, USB charging while off).

Specs that matter:

  • Wattage: continuous rated output. Size for peak system draw plus 20 to 30 percent headroom.
  • Efficiency: 80 PLUS ratings (White, Bronze, Silver, Gold, Platinum, Titanium). A Gold unit wastes less power as heat, which adds up on an always-on server.
  • Connectors: 24-pin ATX, 8-pin EPS for the CPU, PCIe 8-pin or 12V-2x6 for GPUs, SATA power
  • Standard: ATX 3.x units handle modern GPU power spikes
  • Protections: OVP, OCP, OPP, SCP, OTP

Form factors: ATX, SFX for small cases, external bricks for mini PCs and laptops, and hot-swappable redundant PSUs in servers (often paired with separate power circuits).

Failure symptoms: random reboots under load, a system that will not power on, burning smell, or coil whine. An undersized PSU looks exactly like unstable RAM or a bad GPU.

Source: 80 PLUS certification (CLEAResult)

Measuring power on Linux 

# Battery and adapter state on laptops
$ upower -d | grep -E "online|energy-rate"

# CPU package power on Intel and AMD (via RAPL)
$ sudo apt install powertop
$ sudo powertop --time=5

# Server PSU status and readings through the BMC
$ sudo ipmitool sdr type "Power Supply"
PS1 Status       | 0x01 | ok
$ sudo ipmitool dcmi power reading
In the Wild

For a homelab box that runs 24/7, the PSU affects your electricity bill as much as your stability: an efficient unit sized close to real load beats a huge one idling at low efficiency. A kill-a-watt style meter tells you actual draw. When troubleshooting random crashes that survive RAM tests and OS reinstalls, swap the PSU before replacing anything expensive. Put critical gear on a UPS so outages turn into clean shutdowns.

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