What you need to do before you open the case

Replacing a power supply unit (PSU) means removing the old one and installing a new one in your desktop computer. Before you touch anything inside the case, shut down your computer completely and unplug it from the wall outlet. Wait at least 30 seconds — capacitors inside the PSU hold a charge even after power is cut, and touching them can cause injury.

Once the computer has been unplugged and sitting for half a minute, press the power button on the front of the case a few times. This drains any remaining charge. Ground yourself by touching a metal part of the case or wearing an anti-static wrist strap clipped to the case. Static electricity can damage components, so this step matters even though it feels unnecessary.

Before you remove anything, take a photo of how the cables are connected to your motherboard, graphics card, and storage drives. You will need to reconnect them in the same way. If your case has a modular PSU (one where cables plug in rather than being permanently attached), take a photo of which cables go into which ports on the back of the unit.

Key Takeaways

  • Unplug the computer and wait 30 seconds before opening the case, then press the power button to discharge remaining power.
  • Take photos of all cable connections before disconnecting anything so you can reconnect them correctly.
  • The PSU is usually mounted in the bottom or back of the case with four screws holding it in place.
  • Disconnect all cables from the PSU before removing it, then install the new unit and reconnect each cable to its original location.
  • A wattage mismatch — installing a PSU with too little power — will cause the computer to shut down under load or fail to start.

Locating and removing the old power supply

The PSU is a rectangular metal box, usually located in the bottom rear corner of your case. In some cases, especially older or compact ones, it may be mounted at the top. Look for a large fan on one side of the unit — that is the intake. The back of the PSU has a power cord socket, an on/off switch, and a vent.

Unscrew the four bolts holding the PSU to the case. These are usually Phillips-head screws on the outside of the case, near where the PSU sits. Keep the screws in a small container so you do not lose them. Once the screws are out, gently slide the PSU toward the front of the case to loosen it, then disconnect all cables.

The cables you need to disconnect are the 24-pin motherboard power connector (the largest one), the 8-pin CPU power connector near the top of the motherboard, the 6-pin or 8-pin power connectors on your graphics card (if you have one), and the SATA or Molex connectors on your storage drives and fans. Pull straight out on each connector — do not twist or yank at an angle, as this can bend pins or damage the socket.

Choosing the right replacement power supply

The new PSU must have enough wattage to power all your components. A graphics card, multiple drives, and high-end processors all draw power. If your old PSU was 650 watts, a 650-watt replacement will work. If you are unsure of the wattage, look at the label on the back of the old unit — it lists the total wattage in large text.

The new PSU must also fit your case. Most desktop cases accept standard ATX power supplies, but some compact or older cases use smaller units. Check your case manual or measure the space where the old PSU sat. The dimensions are usually listed on the new PSU's box or product page.

The connector types must match. Most modern computers use a 24-pin motherboard connector and an 8-pin CPU connector. If your motherboard is older, it may use a 20-pin connector instead. Check your motherboard manual or look at the connectors on your old PSU to confirm what you need. Modular PSUs (where cables detach) offer flexibility, but non-modular units are usually cheaper and work just as well.

Installing the new power supply unit

Slide the new PSU into the mounting location, aligning the four screw holes with the holes in the case. The fan should face downward if your case has ventilation holes in the bottom, or toward the interior of the case if it does not. Insert the four screws and tighten them firmly but not so hard that you strip the threads.

Plug the 24-pin motherboard connector into the motherboard. This is the largest connector and only fits one way. Push it in until you hear or feel a click. Next, connect the 8-pin CPU power connector near the top of the motherboard — this is usually labeled CPU_PWR or similar on the board itself.

If you have a graphics card, connect the 6-pin or 8-pin power connector to the card. Connect SATA power to your storage drives and any SATA fans. If you have older drives or fans, they may use Molex connectors (the larger, four-pin connectors) instead. Refer to your photos to make sure each cable goes to the correct location.

Testing the new power supply before closing the case

Before you screw the case back together, plug the computer into the wall outlet and flip the power switch on the back of the PSU to the on position. Press the power button on the front of the case. The computer should start up, the fans should spin, and you should see lights on the motherboard and any installed cards.

Let the computer run for a minute or two. Listen for any unusual noises from the new PSU fan. The fan should be quiet or barely audible. If the computer does not start, or if it starts and when ready shuts down, unplug it when ready and check that all connectors are fully seated. A loose 24-pin motherboard connector is the most common cause of this problem.

Once the computer starts and runs without shutting down, shut it down normally through the operating system, unplug it, and close the case. Screw the side panels back on and reconnect your monitor, keyboard, and other peripherals.

Common problems and how to fix them

If the computer does not start at all, the most likely cause is a loose connector. The 24-pin motherboard connector and the 8-pin CPU connector are the most common culprits. Unplug the computer, wait 30 seconds, and reseat both connectors by pulling them out and pushing them back in firmly until they click.

If the computer starts but shuts down after a few seconds or minutes, the PSU may not have enough wattage for your components under load. This is especially common if you upgraded your graphics card or processor since the original PSU was installed. Check the power requirements of your graphics card and processor, add them together, and compare to your PSU wattage. A good rule of thumb is to have at least 100 watts of headroom above your total system draw.

If the new PSU fan is very loud, it may be working harder than it should. This can happen if the PSU is undersized or if the case does not have adequate airflow. Make sure the case has at least one intake fan and one exhaust fan, and that dust filters are clean. If the PSU is the right wattage and the case has good airflow, the fan noise should decrease as the system cools down.

When to replace the power supply versus repair

Power supplies fail in two ways: they stop working entirely, or they work intermittently and cause random shutdowns. In either case, replacement is the correct choice. PSUs are not user-serviceable — opening one can be dangerous, and repair shops charge nearly as much as a new unit costs.

If your computer is shutting down randomly but the PSU fan is spinning and the unit is getting power, the problem might not be the PSU. Overheating, a failing hard drive, or a loose RAM module can cause the same symptoms. Before you buy a new PSU, check that your CPU and graphics card temperatures are normal, that your hard drive is not making clicking sounds, and that all internal components are seated firmly.

Frequently Asked Questions

Can I use a PSU with higher wattage than my computer needs?

Yes. A 750-watt PSU will work fine in a system that only draws 500 watts. The PSU will straightforward not run at full capacity. Higher wattage PSUs are often more efficient and run cooler, so they can actually be a good choice if you plan to upgrade components later.

What if my new PSU has cables I do not need?

If you have a modular PSU, you can leave unused cables unplugged. If you have a non-modular PSU, bundle the unused cables and find them with zip ties or velcro straps to keep them out of the way. Extra cables do not hurt anything as long as they are not blocking airflow.

Do I need to update drivers or BIOS after replacing the power supply?

No. The PSU does not communicate with your operating system or BIOS. Once it is installed and the computer starts, no software changes are needed. The computer will work exactly as it did before.

How long does a power supply usually last?

Most PSUs last five to ten years depending on usage, case temperature, and quality. Higher-end units with better components often last longer. If your PSU is more than seven years old and failing, it is worth replacing even if the wattage is still adequate.

What should I do with the old power supply?

Power supplies contain materials that should not go in the trash. Take the old unit to an electronics recycling center, a computer repair shop that accepts old hardware, or a retailer that offers recycling programs. Many retailers that sell new PSUs will take the old one for free or a small fee.