6 Power Supplies and Power Delivery
Learn how to choose, connect, install, and troubleshoot a computer power supply unit safely and compatibly.
What a PSU does
A computer power supply unit (PSU) converts household AC electricity into regulated DC power for the motherboard, processor, graphics card, drives, and other components. Choosing and installing one correctly means matching its capacity, connectors, and physical dimensions to the system, while handling it safely.
Ratings and cable design
The is the maximum DC output a PSU is designed to provide across its output rails. The rail supplies most of the power used by modern processors and graphics cards; the PSU label lists the available output, often including a combined rating. Do not confuse rated DC output with power drawn from the wall: conversion losses mean wall input is higher.
The indicates tested energy-efficiency performance at specified load levels. A higher efficiency rating can reduce wasted energy and heat, but the label alone does not establish overall build quality, protections, or suitability for a particular PC.
PSUs may be non-modular, with fixed cables; semi-modular, with some detachable cables; or fully modular, with most cables detachable. Modularity affects cable management, not the PSU’s available power. Modular cables are not universally interchangeable: even a cable that fits may have a different pinout. Use only cables specified for the exact PSU model or approved by its manufacturer.
Choose capacity and fit
Estimate system demand from the actual CPU, GPU, motherboard, drives, fans, and other devices. Check component manufacturers’ recommendations and, where useful, a reputable PSU calculator. Allow reasonable headroom for peak loads and planned upgrades. A larger wattage number does not automatically mean a better PSU, and the system does not continuously draw the PSU’s full rated output.
Confirm that the PSU’s and dimensions fit the case. Check its length, cable space, mounting pattern, and the case’s airflow layout as well as the itself.
Identify power connectors
Match each connector to the component and socket it is designed for:
24-pin ATX: Main power connection to most desktop motherboards.
4+4-pin EPS/CPU: Supplies processor power through the motherboard’s CPU-power socket. Some motherboards have an 8-pin socket plus an additional CPU-power socket; follow the motherboard manual.
6+2-pin PCIe: Supplies auxiliary power to graphics cards that use conventional PCIe power sockets.
: 16-pin graphics-card power connectors used on some newer cards and PSUs. The connectors look similar, but the 12V-2x6 specification revised the connector interface. Use a compatible cable or manufacturer-approved adapter, fully seat the plug, and follow the GPU and cable maker’s routing guidance.
SATA power: A flat, keyed power connector used by SATA drives and some accessories.
4-pin peripheral: Often called Molex; an older power connector used by some fans, controllers, and legacy devices.
Connectors that look similar are not necessarily interchangeable. serve different purposes. Match each cable to the socket label and shape, and never force a plug.
Install the PSU safely
Shut down the computer, switch off the PSU, unplug its AC cord, and disconnect external power before opening the case. Never open the PSU enclosure: internal components can retain hazardous charge.
Check the case manual for PSU orientation and airflow. If the PSU fan faces a ventilated opening, keep that opening unobstructed; do not place the fan against a solid panel.
Secure the PSU with the case’s PSU screws. Do not use screws that are too long or force the unit into place.
Route the PSU’s own cables to the correct components. With a modular PSU, connect the PSU-side ends only to sockets and ports identified in that model’s manual.
Insert each connector straight and fully until its latch engages. Connect the motherboard’s main power and CPU power, then any required GPU and drive power. Avoid cable tension and keep cables clear of fans.
Before powering on, check that no loose screw or cable can short a component, and verify that the PSU’s intake and exhaust are not blocked.
For a graphics card with a 12V-2x6 or 12VHPWR connection, fully insert the plug and avoid sharp bends or side pressure immediately at the connector. Follow the specific card and cable instructions. Do not continue using a connector or cable that is damaged, discolored, or unusually hot.
Troubleshoot power problems
If there is no power, check the wall outlet, the AC cord at both ends, and the PSU’s rear switch, usually marked I for on and O for off. Then disconnect power and reseat the 24-pin motherboard connector and CPU-power connector. A PSU fan running is not a reliable test by itself because some models stop the fan at low loads.
If the PC starts and then shuts down or restarts under load, check that the PSU has adequate capacity and the required GPU cables, and that every connector is fully seated. Also consider overheating, a loose connection, a faulty component, or a responding to a fault. These symptoms alone do not prove the PSU is defective.
If a new GPU or other upgrade causes instability, verify the PSU’s rated output and available connectors against the GPU maker’s requirements. Use the number and type of cables specified by the PSU and graphics-card manufacturers; do not rely on a connector merely because it fits.
If a drive or accessory does not start, check that it has PSU power as well as any separate data cable it requires. Inspect the connector and cable for damage. Test with a known-good compatible cable only if the PSU manufacturer confirms compatibility.
If basic checks do not resolve the fault, stop using the computer if there is a burning smell, sparking, visible damage, or a hot or discolored connector. Do not open the PSU or probe its interior; seek qualified repair or replace the unit.