A power supply unit replacement is a straightforward hardware swap that takes 20 to 30 minutes and requires only a screwdriver

Your power supply unit (PSU) converts electricity from your wall outlet into the specific voltages your motherboard, graphics card, and drives need to run. When a PSU fails, your computer will not start, will shut down unexpectedly, or will restart on its own. Replacing it means buying a new unit that matches your system's power needs, opening your case, disconnecting the old cables, removing the mounting bolts, sliding in the new unit, reconnecting the cables, and closing the case again. You do not need to touch any other components or reinstall software.

The entire process is mechanical — no special knowledge of how electricity works is required, and you are unlikely to damage anything if you work slowly and follow each step in order. The most important safety rule is to unplug the computer from the wall before you open the case, and to flip the power switch on the back of the PSU itself to the off position. After that, the job is straightforward disconnecting cables, removing four bolts, sliding the old unit out, sliding the new one in, and reconnecting the same cables in the same places.

Key Takeaways

  • A replacement PSU must have enough wattage for your system — typically 650 to 850 watts for a mid-range gaming computer, or 300 to 500 watts for basic office work.
  • Before you buy, check your current PSU's wattage by looking at the label on the unit itself or in your computer's manual, not by guessing from the case size.
  • Unplug the computer from the wall and flip the power switch on the back of the PSU to the off position before you open the case.
  • The PSU connects to the motherboard with one large 24-pin cable and one smaller 4-pin or 8-pin cable, plus separate cables to each drive and graphics card.
  • After installation, turn on the power switch at the back of the PSU, then press the power button on the front of the case to test that everything works.

Determine the wattage your system needs

The easiest way to find out what wattage you need is to look at the label on your current PSU. Open your case, locate the power supply unit (usually a metal box in the bottom rear corner), and read the number printed on the label — it will say something like "650W" or "750W". Write this number down.

If your current PSU is broken and you cannot read the label, or if you are building a new system, you can estimate based on what you are running. A basic office computer with no graphics card needs 300 to 400 watts. A mid-range gaming computer with a modern graphics card needs 650 to 850 watts. A high-end gaming system with the newest graphics cards needs 850 to 1000 watts or more. When in doubt, choose a PSU with slightly more wattage than you think you need — running at 60 percent capacity is better for the unit's lifespan than running at 95 percent.

Buy a replacement PSU with the right specifications

When you shop for a new power supply, look for one that matches your wattage requirement and fits your case. Most desktop cases accept standard ATX power supplies, which are the most common size. Check your case manual or measure the space where your current PSU sits if you are unsure.

You will see PSUs sold by brand (Corsair, EVGA, Seasonic, and others are common) and by efficiency rating (Bronze, Gold, Platinum, Titanium). A Bronze-rated unit is fine for most people and costs less. Gold-rated units are more efficient and run cooler, but cost more. For a replacement, Bronze or Gold is a reasonable choice. Avoid the cheapest no-name units — a PSU failure can damage other components, so spending a bit more on a known brand is worth it.

Prepare your computer and gather tools

Shut down your computer normally through the operating system. Once it is off, unplug the power cable from the wall outlet. This is the most important safety step — even when the computer is off, the PSU can hold a charge if it is still plugged in.

Locate the power switch on the back of the PSU (a small toggle switch next to where the power cable plugs in) and flip it to the off position. Open your computer case by removing the side panel — usually two or three thumb screws or a latch on the back. You will need a Phillips-head screwdriver to remove the PSU mounting bolts and to disconnect any cables that are held by screws. Have a flashlight or headlamp ready so you can see inside the case clearly, and find a small container or piece of tape to keep the four mounting bolts together so you do not lose them.

Disconnect the old power supply

Before you remove any cables, take a photo of how they are connected. This will help you remember where everything goes when you install the new PSU. Start by disconnecting the large 24-pin cable that runs from the PSU to the motherboard — grip the connector itself (not the wires) and pull straight toward you. Next, disconnect the smaller 4-pin or 8-pin cable that also plugs into the motherboard near the CPU.

Then disconnect the cables that run to your graphics card (if you have one), your hard drives, and your solid-state drives. Most of these connectors pull straight out, but some have small clips on the sides that you need to press before pulling. Work slowly and do not force anything. Once all the cables are disconnected, locate the four bolts that hold the PSU to the case (usually on the back panel, near the bottom). Remove these bolts with your screwdriver and set them aside somewhere you will not lose them. Slide the old PSU out of the case toward the front.

Install the new power supply

Slide the new PSU into the case in the same orientation as the old one — the fan should face downward if your case has ventilation holes in the bottom, or toward the back of the case if it does not. Align the four mounting holes on the PSU with the holes in the case and insert the bolts. Tighten them firmly but do not over-tighten, which can crack the PSU casing.

Now reconnect the cables in reverse order. Start with the large 24-pin motherboard connector — push it straight in until you hear or feel a click. Then connect the 4-pin or 8-pin CPU power cable. Reconnect the cables to your graphics card, hard drives, and solid-state drives, using your photo as a reference. Make sure each connector is fully seated and clicks into place. If a cable does not fit, do not force it — check that you have the right connector type. It is better to spend an extra minute making sure each cable is correct than to plug something in wrong and damage a component.

Test the new power supply

Before you close the case, flip the power switch on the back of the PSU to the on position. Press the power button on the front of your computer. The fans should spin, lights should come on, and the computer should start up normally. If nothing happens, turn off the power switch when ready and check that all cables are fully connected.

Once the computer starts, let it run for a minute or two to make sure it stays on and does not shut down unexpectedly. If everything looks good, shut down the computer normally, unplug it, and close the case. Plug the power cable back into the wall outlet and you are done. Your new PSU is now installed and ready to use.

Frequently Asked Questions

Do I need to replace any other parts when I replace the power supply?

No. A PSU replacement is standalone — you do not need to replace the motherboard, CPU, RAM, or any other component. The only exception is if the PSU failure damaged something else, which is rare. If your computer still will not start after installing a new PSU, the problem may be elsewhere, and you should have it checked by a technician.

What if my new PSU has more wattage than my old one?

That is fine. A higher-wattage PSU will not damage your computer or make it run faster — it just means the unit will run at a lower percentage of its capacity, which is actually better for its lifespan. Your computer will only draw the power it needs.

Can I reuse the cables from my old power supply with the new one?

Only if both PSUs are from the same manufacturer and the same product line. Different PSU models can have different pin layouts on the connectors, even if they look similar. Using the wrong cables can damage your components. It is safest to use the cables that come with your new PSU.

How do I know if my PSU is actually broken or if something else is wrong?

If your computer will not start at all and the fans do not spin, the PSU is likely the problem. If the computer starts but shuts down after a few minutes, or restarts randomly, it could be the PSU or it could be a motherboard or CPU issue. A technician can test the PSU with a multimeter to confirm whether it is working.

Should I keep my old power supply after I replace it?

If it is broken, take it to an electronics recycling center — do not throw it in the trash. If it still works, you can keep it as a spare or sell it. A working used PSU has value and can be useful if you build another computer later.