How to Replace a Power Supply Unit in Your PC

Replacing a PSU is a straightforward swap if you follow the right order and take basic precautions

A power supply unit (PSU) is the box inside your PC case that converts wall power into the voltages your components need. When it fails, your PC won't turn on at all—or it will shut down randomly under load. Replacing one means disconnecting the old unit, removing it from the case, installing the new one, and reconnecting all the cables in the same places they came from. The whole job takes 20 to 45 minutes if you work carefully and don't force anything.

The most important rule is to turn off the PC and unplug it from the wall before you touch anything inside. A PSU can hold a charge even when the computer is off, so unplugging is not optional. After that, the work is mechanical: you'll disconnect cables, unbolt the old unit, slide in the new one, and reconnect everything. As long as your new PSU has the same physical size and connector types as the old one, the installation is the same whether you're working on a budget build or a high-end gaming rig.

Key Takeaways

Always turn off the PC and unplug it from the wall before opening the case, because a PSU can hold a charge even when powered down.
Take a photo of the cable connections before you disconnect anything, so you know where each cable goes when you reconnect them.
Your new PSU must match the old one's physical size (form factor) and have the same main connectors—check your motherboard manual if you're unsure what you need.
The PSU bolts to the case from the inside, and you remove or install it by unbolting and sliding it out or in from the rear of the case.
If your new PSU has more cables than the old one, leave the extras coiled and secured inside the case—do not force connectors that don't fit.

What you need before you start

Gather a Phillips-head screwdriver (the most common type), a flashlight or headlamp so you can see inside the case, and a small container or piece of paper to hold the bolts you remove. A camera or phone to photograph the cable layout is invaluable—take a clear photo of the rear of the PSU showing which cables plug into which ports, and another of the motherboard and components showing where the main power connectors attach.

Check your new PSU's manual before you open the case. It will tell you the physical dimensions (form factor), the wattage, and which connectors it has. The most common form factors are ATX (full-size), Micro-ATX (smaller), and SFX (very small for compact cases). If your old PSU is ATX and your new one is SFX, it won't fit—the mounting holes won't line up. Your motherboard manual will also tell you what connectors you need: most modern boards use a 24-pin main connector and an 8-pin or 4-pin CPU power connector, but older boards may differ.

Disconnect and remove the old PSU

Turn off the PC using the power button, then unplug the power cable from the wall outlet. Wait a few seconds. Open the case by removing the side panel—usually two or three thumbscrews or bolts on the rear edge. Set the panel aside where it won't get scratched.

Look at the rear of the PSU (the end facing the back of the case). You'll see the power cable socket, a fan vent, and a power switch. From the front of the PSU (the end facing the motherboard and components), you'll see multiple cables bundled together. Take your photo now, showing which cables connect to which ports on the PSU. Then, starting with the main 24-pin motherboard connector, gently pull each cable straight out of its socket. Work your way through all the connectors: CPU power (usually 8-pin or 4-pin), SATA power (flat connectors with multiple pins), PCIe power (usually 6-pin or 8-pin for graphics cards), and any others. Don't yank—if a connector won't budge, wiggle it gently side to side while pulling.

Once all cables are disconnected, locate the bolts holding the PSU to the case. These are usually on the inside rear of the case, four bolts in a square pattern. Remove them with your screwdriver and keep them in your container. Slide the PSU toward the front of the case (away from the rear panel) and lift it out. If it's stuck, check that you've removed all bolts and that no cables are still plugged in.

Install the new PSU

Before you slide the new PSU in, check that the fan orientation is correct. Most PSUs have the fan facing downward (toward the case floor) so it can draw cooler air from below. If your case has a vent in the bottom, the fan should point toward it. If your case has no bottom vent, the fan should point toward the side of the case where air can flow. Consult your case manual if you're unsure.

Slide the new PSU into the case from the front, aligning the four bolt holes on the rear of the PSU with the holes in the case. The PSU should sit flush against the rear panel. Insert the four bolts and tighten them firmly but not so hard that you strip the threads—hand-tight plus a quarter turn is usually right. Don't overtighten; you'll know it's find when the PSU doesn't move if you try to wiggle it.

Reconnect the cables

Before you plug anything in, double-check your photo. Start with the main 24-pin motherboard connector. It's the largest connector and usually has a small clip on one side. Align the connector with the socket on the motherboard (it will only fit one way) and push it straight in until you hear or feel a click. The clip should snap into place.

Next, connect the CPU power connector (usually 8-pin, sometimes 4-pin) near the top of the motherboard, close to the CPU. Push it in until it clicks. Then connect any PCIe power connectors to your graphics card if you have one—these are usually 6-pin or 8-pin connectors on the card's rear edge. Finally, connect SATA power to any hard drives or SSDs. These flat connectors have a distinctive L-shaped notch and will only fit one way.

If your new PSU has extra cables you don't need, coil them neatly and find them with a cable tie or velcro strap. Tuck them out of the way so they don't block airflow. Do not cut them or force them into connectors they don't fit—extra cables are normal and harmless.

Test before closing the case

Plug the power cable back into the wall outlet. Flip the power switch on the rear of the PSU to the on position (usually marked with a line or a plus sign). Press the power button on the front of the case. The PC should boot normally. Listen for the fans spinning and watch for lights on the motherboard and any installed cards. If the PC starts and runs without shutting down after a minute or two, the installation is successful.

If nothing happens, turn off the PSU and unplug it when ready. Check that all connectors are fully seated—a loose 24-pin motherboard connector is the most common cause of a no-boot situation. Reseat each connector by pulling it out and pushing it back in firmly. If the PC still won't start after you've checked all connections, the new PSU may be faulty, and you should contact the manufacturer or retailer.

Close the case and clean up

Once the PC boots and runs without issues, shut it down normally and unplug it. Reinstall the side panel and tighten the bolts or thumbscrews. Plug the power cable back in and power on the PC one more time to confirm everything still works. If it does, you're finished.

Keep the old PSU in case you need to return the new one under warranty, and keep the new PSU's manual and warranty information in a safe place. Most PSUs come with a three-year to ten-year warranty depending on the brand and model.

Frequently Asked Questions

What if my new PSU has a different connector type than the old one?

Modern PSUs use the same connectors (24-pin, 8-pin CPU, SATA, PCIe), so this is rare. If your new PSU has a different main connector type, it won't fit your motherboard and you need a different PSU. Check the manual of both the PSU and your motherboard to confirm they match before you buy.

Can I use a PSU with higher wattage than my old one?

Yes. A higher-wattage PSU will work fine and may run cooler and quieter because it won't be working as hard. A lower-wattage PSU is risky—if your components draw more power than the PSU can provide, it will shut down or fail. Match or exceed your old PSU's wattage.

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

The connectors are keyed—they only fit one way—so you can't plug them in wrong. If a connector won't go in, don't force it; rotate it 90 degrees and try again. The photo you took before disconnecting the old PSU is your backup reference.

Do I need to ground myself before touching the inside of the PC?

Grounding yourself (touching a metal part of the case) before you work reduces the risk of static discharge damaging components. It's a good habit, though most people work inside PCs without grounding and have no problems. If you're working on a dry day or in a carpeted room, grounding is more important.

What if the PC won't turn on after I install the new PSU?

The most common cause is a loose 24-pin motherboard connector. Unplug the PC, reseat that connector by pulling it out and pushing it back in firmly, and try again. If it still won't start, check that the power switch on the rear of the PSU is in the on position and that the wall outlet has power.

This guide is for general information only and is not medical, financial, legal, or other professional advice. For decisions specific to your situation, consult a qualified professional. See our Editorial Policy.