What You Need to Do Before Opening Your Computer

Replacing a power supply unit (PSU) is a straightforward task that takes 20 to 40 minutes and requires no special skills—just patience and attention to detail. Before you touch anything inside your computer, shut it down completely, unplug the power cable from the wall outlet, and wait five minutes. This matters because capacitors inside the PSU can hold a charge even when the computer is off, and touching them can cause injury.

Next, ground yourself to avoid damaging components with static electricity. Touch a metal part of the case with your bare hand, or wear an anti-static wrist strap clipped to the case. These straps cost $5 to $10 and are optional but recommended if you are replacing a PSU in an expensive computer. Open the case according to your computer's manual—usually two or four screws on the back panel—and set it on a clean, flat surface where you have room to work.

Key Takeaways

  • Always shut down the computer, unplug it from the wall, and wait five minutes before opening the case to avoid electrical shock.
  • Take a photo of the cable connections before unplugging anything, so you know where each cable goes when you reinstall the new PSU.
  • The PSU is usually a metal box in the back corner of the case held in place by four screws on the outside of the case.
  • New power supplies cost $50 to $200 depending on wattage and quality; match the wattage of your old PSU unless you are upgrading your graphics card or processor.
  • Plug in the new PSU, flip the power switch to ON, and test the computer before closing the case.

Locating and Photographing Your Current Connections

The power supply is a metal box, usually rectangular, mounted in the back corner of your case—often the bottom rear corner, but sometimes the top. Look for a large bundle of cables running from it to your motherboard, graphics card, and storage drives. Before you disconnect anything, take a clear photo of how the cables are plugged in. Use your phone camera and make sure the image is sharp enough to read the labels on the connectors.

This photo is your safety net. Even experienced builders refer back to photos because PSU connectors look similar but fit only in the correct slots. Plugging a cable into the wrong connector can damage your motherboard or graphics card. If your PSU has a manual, keep it nearby—it will show you which connector is which.

Removing the Old Power Supply

Locate the four screws holding the PSU to the case. They are on the outside back panel, usually arranged in a square pattern around where the PSU's fan is visible. Use a Phillips-head screwdriver to remove all four screws and set them aside in a small container so you do not lose them. Do not force the screws—if one is stuck, explore a small amount of penetrating oil and wait a minute.

Now unplug every cable from the PSU. Start with the large 24-pin connector that plugs into the motherboard, then the 8-pin or 4-pin CPU power connector near the processor. Next, unplug any 6-pin or 8-pin connectors from your graphics card if you have one. Finally, unplug the SATA power connectors from your hard drives or SSDs. Gently pull the PSU toward the front of the case and lift it out. It will be heavy—usually 5 to 10 pounds—so use both hands.

Installing the New Power Supply

Before you install the new PSU, check its wattage rating on the label. It should match your old PSU's wattage unless you are upgrading your graphics card or processor, in which case you may need more power. Common wattages are 500W, 650W, 750W, and 850W. If you are unsure, a 650W PSU works for most desktop computers built in the last ten years.

Slide the new PSU into the case with the fan facing downward if there is a vent in the case bottom, or facing into the case if there is no vent. Align the four screw holes on the PSU with the holes in the case and insert the four screws. Tighten them firmly but do not overtighten—you should feel resistance, not a grinding sensation. Now plug in the 24-pin motherboard connector first, then the CPU power connector, then any graphics card connectors, and finally the SATA power connectors to your drives. Refer to your photo to make sure each cable goes into the correct slot.

Testing Before You Close the Case

Plug the power cable back into the wall outlet. Flip the power switch on the back of the PSU to the ON position—you should see a small light on the PSU itself. Press the power button on the front of your computer. The fans should spin, lights should come on, and you should hear the normal startup sounds. Let the computer run for two to three minutes and watch for any unusual noises, smells, or smoke. If everything looks normal, shut down the computer using the normal shutdown process, not the power button.

Once the computer is fully shut down, close the case and screw it back together. Power on the computer one more time to confirm it starts normally. If the computer does not start, shut it down when ready, unplug it, and check that all cables are seated firmly in their connectors. A loose cable is the most common reason a computer will not start after a PSU replacement.

Common Mistakes to Avoid

The biggest mistake is plugging a cable into the wrong connector. The 24-pin motherboard connector and the 8-pin CPU connector look similar but are different sizes and will not fit if you try to force them. If a connector does not slide in smoothly, stop and check your photo. Do not use force.

Another common error is forgetting to flip the power switch on the back of the PSU to ON before pressing the power button on the front of the computer. The computer will not start if this switch is in the OFF position, and many people think the PSU is broken when it is just switched off. A third mistake is leaving cables loose or pinched under the case. Loose cables can cause intermittent power loss, and pinched cables can overheat and damage the insulation. Route cables neatly and make sure they are not bent sharply or pressed against sharp edges inside the case.

When to Buy a New PSU vs. Repair

Power supplies cannot be repaired at home. If your PSU is making a loud humming noise, your computer is shutting down randomly, or your PSU is more than seven years old, replacement is your only option. A new PSU costs $50 to $200 depending on wattage and brand. Budget PSUs from brands like Corsair, EVGA, and Seasonic are reliable and reasonably priced. Avoid the cheapest options—a failing PSU can damage your motherboard, so spending an extra $30 to $50 on a quality unit is worth it.

If your computer is under warranty, check whether the PSU is covered before you buy a replacement. Some manufacturers will replace a failed PSU for free or at a reduced cost. Contact the computer maker's support line with your serial number to find out.

Frequently Asked Questions

Do I need to buy the exact same wattage as my old PSU?

No, but you should match it or go higher. If your old PSU was 650W, a new 650W or 750W PSU will work fine. Going lower—for example, replacing a 750W with a 550W—can cause your computer to shut down under heavy load. If you are upgrading your graphics card or processor, you may need more wattage; check the power requirements of your new components.

Can I test the new PSU before installing it in the case?

Yes. Plug it in, flip the power switch to ON, and listen for the fan to spin. You should hear a brief whirring sound. Do not plug any cables into it yet—just test that it powers on. This takes 10 seconds and confirms the PSU itself is working before you install it.

What if my computer starts but shuts down after a few minutes?

This usually means a cable is loose or not fully seated. Shut down the computer, unplug it, and check that the 24-pin motherboard connector and the 8-pin CPU connector are pushed all the way in until you hear a click. Reseat any SATA power connectors as well. Plug the computer back in and test again.

Is it safe to replace a PSU myself, or should I take it to a repair shop?

It is safe to do yourself if you follow the steps in order and take your time. The main risk is static electricity, which you can prevent by grounding yourself. If you are uncomfortable opening your computer, a repair shop will charge $50 to $100 in labor to replace the PSU, on top of the cost of the unit itself.

How do I know what wattage PSU I need?

Check the label on your old PSU—it will say something like "650W" or "750W". If your old PSU is broken and you cannot read the label, look up your computer model online or add up the power requirements of your processor, graphics card, and storage drives. Most modern computers use 550W to 750W. When in doubt, 650W is a safe choice for most builds.