What You Need to Do Before Opening Your Computer

Replacing a PC power supply means removing the old unit and installing a new one in its place. The process takes 20 to 40 minutes for most people and requires only a screwdriver and basic care around electrical components. Before you touch anything inside your case, shut down your computer completely, unplug the power cable from the wall outlet, and wait five minutes — this drains residual electricity that can damage parts or harm you.

Ground yourself by touching a metal part of the case or wearing an anti-static wrist strap clipped to the case. Static electricity can destroy components silently, so this step matters even though you cannot see it happening. If your case has a power switch on the back (most do), flip it to the off position as well.

Key Takeaways

  • Always unplug the power cable from the wall and wait five minutes before opening your case, then ground yourself by touching the metal frame.
  • Take a photo of the cable connections before unplugging anything so you know where each cable goes when you reinstall.
  • The power supply sits in the bottom or back corner of your case and is held by four screws on the outside; remove those screws and slide the unit out.
  • New power supplies connect to the motherboard, CPU, graphics card, and storage drives in the same way the old one did — match the connector shapes, not colors.
  • Test the new power supply by plugging in the monitor and keyboard, powering on, and checking that fans spin and lights appear before reconnecting everything else.

Locating and Removing the Old Power Supply

Open your case by removing the side panel — usually two or four thumbscrews or regular screws on the back edge. Set the panel aside in a safe place. Look inside and find the power supply, which is a metal box roughly the size of a toaster. In most cases, it sits in the bottom rear corner, but some cases mount it at the top or on the side. The fan opening faces either down or toward the back of the case.

Before you disconnect anything, take a clear photo of all the cables plugged into the power supply. This photo is your map for reconnecting the new one. You can also sketch it on paper, but a photo is faster and more accurate. Unplug each cable from the power supply itself — not from the components. The cables stay in the case; only the power supply comes out.

Locate the four screws on the outside of the case that hold the power supply in place. These are usually on the back panel, near where the power supply sits. Remove all four screws and set them aside where you will not lose them. Gently slide the power supply toward the front of the case (or in whichever direction it came in) and lift it out. The unit may be heavy, so use both hands.

Choosing the Right Replacement Power Supply

The new power supply must have enough wattage to run your system. Check your current system's power draw by looking at your graphics card and CPU specifications — a tool like PCPartPicker can estimate total wattage if you enter your components. As a rule, buy a power supply rated 20 to 30 percent higher than your estimated need. If your system draws 450 watts, a 550 or 650-watt unit is safer than a 500-watt one.

The power supply must also fit your case. Check your case manual or the case itself for the form factor — most common are ATX (full-size), Micro-ATX (smaller), and SFX (very small). The power supply box will state its form factor clearly. An ATX power supply will not fit a case designed for SFX, so verify this before buying.

Look for a power supply with an 80+ Bronze certification or higher — this means it meets efficiency standards and will waste less energy as heat. Brands like Corsair, EVGA, Seasonic, and Be Quiet are widely available and reliable. Avoid the cheapest no-name units, as they fail more often and can damage your components.

Installing the New Power Supply

Unbox the new power supply and inspect it for damage. Place it in the same position where the old one sat, with the fan facing the correct direction (usually down or toward the back). Align the screw holes on the back of the case with the holes on the power supply and insert the four screws. Tighten them firmly but do not over-tighten — you want them snug, not stripped.

Now reconnect the cables using your photo as a guide. The main connectors are shaped differently so they only fit one way — you cannot plug a CPU power cable into a SATA connector by mistake. Match the shape of the connector to the shape of the port. The main 24-pin connector goes to the motherboard, the 8-pin or 4-pin connector goes to the CPU, the 6-pin or 8-pin connectors go to the graphics card (if you have one), and the L-shaped SATA connectors go to storage drives and other devices.

Do not force any connector. If it does not slide in smoothly, check the orientation and try again. Forcing a connector can bend pins and ruin both the cable and the component. Once all cables are connected, double-check against your photo to make sure nothing is missed.

Testing Before You Close Everything Up

Before you screw the side panel back on, test the new power supply. Plug the power cable into the wall outlet and into the back of the power supply. Plug in your monitor and keyboard (leave other devices unplugged for now). Flip the power switch on the back of the power supply to the on position and press the power button on your case.

Listen and watch for the fans to spin — both the power supply fan and the case fans should turn. Lights on the motherboard or case should appear. If the system powers on and the fans run, the power supply is working. If nothing happens, turn everything off when ready, unplug the power cable, and check that all connectors are fully seated.

Once you confirm the system powers on, shut it down and unplug the power cable. Now reconnect the rest of your devices — additional storage drives, USB hubs, or other peripherals — and close the side panel. Plug everything back in and power on for a full test. If the system boots to your operating system and runs normally, the replacement is complete.

Common Mistakes to Avoid

The most common error is forgetting to unplug the power cable before opening the case. Even with the computer shut down, electricity can flow through the power supply if it is still plugged in. Always unplug first and wait a few minutes.

Another frequent mistake is not taking a photo of the cable connections before unplugging them. Without a reference, it is straightforward to plug cables into the wrong ports, which can damage your motherboard or graphics card. Spend 30 seconds on a photo — it saves hours of troubleshooting.

Forcing connectors is also common and damaging. If a connector does not slide in smoothly, stop and check the orientation. Bent pins on a motherboard or graphics card are expensive to repair. The correct connector should fit with gentle, steady pressure.

Finally, do not skip the test step. Powering on with just the monitor and keyboard connected lets you catch problems before you reconnect everything. If something is wrong, you can troubleshoot quickly without having to open the case again.

What to Do With Your Old Power Supply

Do not throw the old power supply in the trash. Power supplies contain materials that are harmful to the environment if they end up in a landfill. Take it to an electronics recycling center, a computer repair shop that accepts old parts, or a retailer like Best Buy that offers recycling programs. Some recyclers will even pay you a small amount for the scrap metal and components inside.

If the old power supply still works, you can also donate it or sell it online if you want to recover some cost. Make sure to note its wattage and condition in any listing so the buyer knows what they are getting.

Frequently Asked Questions

Can I replace the power supply while the computer is just turned off but still plugged in?

No. Always unplug the power cable from the wall outlet. A powered-off computer can still have electricity flowing through the power supply, which can shock you or damage components. Unplug, wait five minutes, then open the case.

What if the new power supply does not fit in the case?

Check that you bought the correct form factor for your case — ATX, Micro-ATX, or SFX. If you bought the right size and it still does not fit, the case may have internal obstructions. Measure the space and compare it to the power supply dimensions before returning it. You may need a different case or a smaller power supply.

Do I need to install drivers for the new power supply?

No. A power supply does not require drivers. It is a hardware component that provides power; your operating system does not need to recognize it or install software for it. If your computer does not power on after installation, the problem is a loose cable or incorrect connection, not a missing driver.

How do I know if my power supply is failing before it stops working completely?

Signs of a failing power supply include random shutdowns, the computer not powering on on the first try, burning smells, or fans that spin very loudly. If you notice any of these, the power supply may fail soon. Replacing it before it dies completely prevents data loss and damage to other components.

Can I use a power supply from a different brand than my current one?

Yes. Power supplies from different brands work the same way and use the same connectors. As long as the new power supply has enough wattage and fits your case, the brand does not matter. You can replace a Corsair with an EVGA, or any other combination.