What You Need to Do Before You Start

Replacing a power supply means removing the old unit from your case and installing a new one in its place. The job takes 15 to 30 minutes and requires only a screwdriver. You do not need to touch the motherboard, CPU, or any other internal component — just the power supply itself.

Before you open the case, shut down your computer completely and unplug it from the wall outlet. Wait five minutes. Even after unplugging, a power supply holds a small electrical charge, and waiting gives it time to discharge. This step matters: touching the wrong part of a live power supply can cause injury.

Gather a Phillips-head screwdriver, a flashlight or headlamp, and the new power supply unit. Check the new unit's wattage and connector type against your old one — most modern PCs use 24-pin motherboard connectors and 8-pin CPU connectors, but older systems may differ. If you are unsure, look up your motherboard model online or check the manual that came with your PC.

Key Takeaways

  • Unplug your PC and wait five minutes before opening the case to let the power supply discharge safely.
  • Remove the four screws holding the power supply to the case, then disconnect every cable from the motherboard, CPU, and storage drives before lifting the unit out.
  • Slide the new power supply into the case with the fan facing down (in most cases), align the screw holes, and fasten it with the four screws that came with the new unit.
  • Reconnect each cable to its original port — 24-pin to motherboard, 8-pin to CPU, and SATA or Molex connectors to drives — then plug the power cable back into the wall and test the system.

Disconnect All Power Cables Inside the Case

Open your PC case by removing the side panel. You will see the power supply mounted in the bottom rear corner, with cables running from it to the motherboard, CPU, graphics card (if present), and storage drives. Take a photo of the cable layout with your phone — this makes reconnection much faster and reduces the chance of plugging something into the wrong port.

Locate the large 24-pin connector plugged into the motherboard. Press the small plastic tab on the connector and pull straight out. Do the same for the 8-pin CPU power connector near the top of the motherboard. If your system has a graphics card, find the 6-pin or 8-pin power connector on the card itself and disconnect it. For storage drives, disconnect any SATA power connectors (the L-shaped white or black plugs) or older Molex connectors (the larger rectangular plugs with four pins).

As you disconnect each cable, you can coil it loosely and set it aside, or leave it plugged into the power supply for now — you will remove the whole unit in the next step. Take your time and make sure every cable is free before proceeding.

Remove the Old Power Supply from the Case

Locate the four screws holding the power supply to the case. In most PC cases, these are on the rear panel, visible from outside the case. Use your screwdriver to remove all four. Keep the screws in a small container so you do not lose them — you will use them again.

Once the screws are out, gently slide the power supply toward the front of the case. It should move freely. Lift it straight up and out of the case. Set it on a flat surface away from your work area. Do not force it — if it catches on a cable or component, stop and check what is blocking it before pulling harder.

Install the New Power Supply

Before placing the new unit in the case, check the orientation of the fan. In most modern cases, the fan should face downward toward the case floor so it can draw cool air from outside. Some older cases have the fan facing upward. Look at how the old unit was oriented and match that position with the new one.

Slide the new power supply into the case, aligning the screw holes on the rear panel with the holes in the case. The unit should sit flush against the case wall. Insert the four screws and tighten them firmly but not so hard that you strip the threads. The power supply should not move or rattle when you gently push on it.

Reconnect All Power Cables

Using the photo you took earlier, reconnect each cable to its original port. Start with the 24-pin motherboard connector — it is the largest 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. If your graphics card needs power, reconnect its 6-pin or 8-pin connector. Finally, reconnect SATA or Molex power to your storage drives.

Double-check each connection by gently tugging on the connector — it should not come loose. Loose connections are the most common cause of boot failures after a power supply replacement. If a connector feels loose, remove it and reinsert it firmly until it clicks.

Test the System Before Closing the Case

Close the case but do not screw the side panel back on yet. Plug the power cable into the wall outlet and flip the power supply switch to the on position (the switch is usually on the rear of the unit). Press the power button on your PC. The system should boot normally, fans should spin, and lights should come on.

Let the system run for two to three minutes. Listen for any unusual sounds — a grinding noise or loud buzzing usually means something is wrong. If the system boots and runs quietly, shut it down, unplug it, and close the case. If the system does not boot or makes strange sounds, unplug it when ready, check that all cables are seated firmly, and try again.

Frequently Asked Questions

Do I need to turn off the power supply switch before unplugging the PC?

Yes. Flip the switch on the rear of the power supply to the off position, then unplug the cable from the wall. This ensures no power is flowing through the unit while you work on it.

What if my new power supply has extra cables I do not need?

Leave them disconnected and coil them neatly inside the case or bundle them with a cable tie. Do not force unused connectors into ports — they will not fit and you risk damaging the connector or the port.

Can I reuse the screws from the old power supply?

Yes, if they are in good condition. However, most new power supplies come with their own screws. Use the new ones if they are included — they are matched to the unit and less likely to cause problems.

How do I know if the new power supply is the right wattage?

Check your old power supply's label or your PC's documentation. A new supply should match or exceed the old one's wattage. If you are unsure, look up your motherboard model and add up the power requirements of your CPU and graphics card — most modern systems need between 450 and 750 watts.

What if the PC still will not start after I reconnect everything?

Unplug the system and check every cable connection again, especially the 24-pin motherboard connector and the 8-pin CPU connector. These are the most common sources of boot failures. Reseat each one by removing it and pushing it back in firmly until it clicks. If the system still does not start, the new power supply may be faulty — contact the manufacturer or retailer.