Replacing a PSU means unplugging the old one, disconnecting its cables from your components, sliding it out, and installing a new one in the same spot

A power supply unit (PSU) is the metal box inside your computer case that converts wall power into the right voltage for your motherboard, graphics card, and storage drives. When one fails, your computer won't start at all—or it starts and shuts down randomly. Replacing it is a straightforward job that takes 20 to 40 minutes and requires only a Phillips screwdriver and a small amount of care with cable connections.

The hardest part is usually finding the right replacement PSU—one with enough wattage for your components and the right connector types for your motherboard. Once you have the new unit, the physical swap is mechanical: you'll turn off and unplug the old one, disconnect four to eight cables, remove four bolts, slide the new one in, reconnect those same cables, and bolt it down.

Key Takeaways

  • Always turn off your computer and unplug the power cable from the wall before touching any internal components or cables.
  • Write down or photograph which cables connect to which components before you disconnect anything, so you can reconnect them correctly.
  • The PSU bolts to the rear wall of the case from the inside, and you remove them from inside the case—not from outside.
  • Most modern motherboards use a 24-pin main connector and an 8-pin CPU power connector; graphics cards use 6-pin or 8-pin connectors.
  • A replacement PSU should have at least as much wattage as your current one, and ideally 20 to 30 percent more if you plan to upgrade components later.

Gather your tools and check the old PSU's specifications

Before you open the case, you need to know what you're replacing. Look at the sticker on the side or back of your current PSU—it will show the wattage (usually 450W, 550W, 650W, 750W, or higher) and the model number. Write this down or take a photo. You'll also see the input voltage (should say 100–240V, which works in any country) and the output rails—this tells you the PSU's design, but the wattage number is what matters most for your replacement.

Gather a Phillips screwdriver (medium-sized), a flashlight or headlamp if your case is dark inside, and a small notepad or your phone's camera. Some people also keep a grounding strap nearby—a wrist strap that clips to the case to prevent static electricity from damaging components—but it's not required if you touch the metal case frequently while working.

Power down and disconnect all cables from the PSU

Turn off your computer using the normal shutdown process. Wait 30 seconds for the system to fully power down, then flip the power switch on the back of the PSU to the OFF position (if your PSU has one). Unplug the power cable from the wall outlet and from the back of the PSU.

Now open your computer case. Most cases have two side panels held by thumbscrews or regular screws; remove whichever side gives you access to the PSU. The PSU is usually in the bottom rear corner of the case. Before you touch anything, press your hand against the metal case frame to discharge any static electricity from your body.

Disconnect the power cables one at a time and label them

The PSU has a thick bundle of cables running to your motherboard, graphics card, storage drives, and case fans. Take a photo of the entire cable layout before you disconnect anything—this is your reference if you forget where a cable goes. Then, starting with the largest connector (the 24-pin motherboard power), gently pull the connector straight out of its socket. Do not twist or yank; connectors are designed to slide straight out.

Work through each cable: the 8-pin CPU power (near the top of the motherboard), any 6-pin or 8-pin graphics card power connectors, SATA power connectors (flat, L-shaped plugs for hard drives and SSDs), and Molex connectors (larger, four-pin plugs for older fans or drives). As you disconnect each one, you can either label it with tape and a pen or straightforward remember that each connector is keyed—it only fits one way, so you cannot plug it in backward. Take your time and do not force any connector.

Remove the four bolts holding the PSU to the case

Once all cables are disconnected, locate the four bolts on the rear wall of the case that hold the PSU in place. These bolts are on the inside of the case, not the outside—you access them from inside the case. They are usually Phillips-head and sit in the four corners of the PSU's mounting bracket. Use your screwdriver to remove all four bolts and set them aside in a small container so you don't lose them.

Gently slide the PSU toward the front of the case (away from the rear wall) and lift it out. It is heavier than it looks—usually 5 to 8 pounds—so use both hands and keep your back straight. Set the old PSU aside on a flat surface.

Install the new PSU and reconnect all cables

Position the new PSU in the same spot, with the fan facing down (if your case has ventilation holes in the bottom) or up (if it doesn't). Align the four bolt holes on the PSU with the mounting holes on the case. Insert the four bolts and tighten them by hand first, then use your screwdriver to snug them down—do not over-tighten, as you can strip the threads.

Now reconnect the cables in reverse order. Start with the 24-pin motherboard connector—line it up with the socket and push it straight in until you hear or feel a click. Then connect the 8-pin CPU power, the graphics card power (if you have a dedicated GPU), and finally the SATA and Molex connectors for drives and fans. Use your photo as a reference if you're unsure. Each connector is keyed, so it will only fit the right way; if it does not slide in smoothly, stop and check the alignment rather than forcing it.

Test the new PSU before closing the case

Plug the power cable back into the wall outlet and into the rear of the new PSU. Flip the power switch on the PSU to the ON position. Press the power button on your computer—it should start normally. Listen for the fan inside the PSU to spin up, and watch for lights on the motherboard and any case LEDs to turn on. If the computer starts and runs for 30 seconds without shutting down, the PSU is working correctly.

If nothing happens, turn off the PSU when ready and check your cable connections. The most common mistake is a loose 24-pin motherboard connector or a 8-pin CPU power connector that is not fully seated. Reseat each connector by pulling it out slightly and pushing it back in firmly. If the computer still does not start after checking all connections, the new PSU may be defective—contact the manufacturer or retailer.

Once the computer runs without issue, shut it down normally, unplug the power cable, and close the case. Reinstall the side panel and any screws you removed, and you're done.

Common mistakes to avoid

The most frequent error is forgetting to disconnect the 8-pin CPU power connector before trying to remove the PSU—this connector sits near the top of the motherboard and is straightforward to miss. Another common mistake is plugging the 24-pin connector in only halfway; it should click and sit flush with the socket, not stick out at an angle.

Some people also buy a PSU with too little wattage, thinking their current wattage is enough. If your old PSU was 450W and you're replacing it, a 450W replacement will work, but a 550W or 650W unit gives you headroom for future upgrades and runs cooler under load. Do not buy a PSU with less wattage than your current one unless you have a specific reason.

Finally, avoid touching the cables or connectors with wet hands, and do not work on your computer during a thunderstorm—a power surge can damage the new PSU before you even finish installing it.

Frequently Asked Questions

Do I need to turn off the computer before unplugging the PSU?

Yes. Always shut down the operating system first, wait for the computer to fully power off, then flip the PSU's power switch to OFF and unplug the wall cable. Pulling the power cable while the computer is running can corrupt files and damage components.

What if I don't remember which cable goes where?

Take a photo of the entire cable layout before you disconnect anything. Each connector is also keyed—shaped so it only fits one way—so you cannot plug it in backward. If a connector does not slide in smoothly, check the alignment and try again rather than forcing it.

Can I replace the PSU while the computer is on?

No. Always power down completely, unplug the wall cable, and wait 30 seconds before touching any internal components. Working on a live PSU is dangerous and can destroy your hardware.

How do I know what wattage PSU to buy?

Check the sticker on your current PSU for its wattage rating. A replacement should have at least the same wattage; 20 to 30 percent more is ideal if you plan to upgrade your graphics card or CPU later. If you're unsure, look up your motherboard and graphics card models online to see their recommended PSU wattage.

What if the new PSU doesn't start the computer?

Turn off the PSU when ready and check that the 24-pin motherboard connector and the 8-pin CPU power connector are fully seated—they should click and sit flush. Reseat each connector by pulling it out slightly and pushing it back in firmly. If the computer still does not start, the PSU may be defective and should be returned.