A power supply unit (PSU) is replaceable, and the job takes about 30 minutes if you follow the steps in order

Your PSU sits in the bottom or back corner of your case and connects to every major component inside — the motherboard, graphics card, storage drives, and fans. When it fails, your computer stops turning on. The good news is that replacing it does not require special tools or technical training. You disconnect the cables, unbolt the old unit, slide in the new one, reconnect the cables, and power on. The hardest part is choosing the right replacement, not the physical work.

This guide walks you through both: how to pick a PSU that matches your system, and how to physically swap it out. You do not need to be an electrician or have built a computer before. You do need to be comfortable working inside your case and following a sequence of steps without skipping ahead.

Key Takeaways

  • Your replacement PSU must have enough wattage for your components — usually 80 to 100 watts more than your system's peak draw, which you can estimate from your CPU and graphics card specs.
  • The physical replacement takes about 30 minutes: unplug the computer, disconnect all cables from the old PSU, unbolt it, slide the new one in, reconnect cables in the same order, and test.
  • Cable connectors are keyed so they only fit one way — if a connector does not slide in smoothly, you are pushing it at the wrong angle, not explore too little force.
  • Take a photo of the cable layout before you disconnect anything, so you know where each connector goes when you reconnect.
  • A PSU with 80 Plus Bronze certification or higher is more efficient and runs cooler than an unrated unit, which saves money over time.

Choosing the right wattage for your system

The wattage rating on a PSU tells you the maximum power it can deliver. If your system draws more than that, the PSU shuts down to protect itself — your computer cuts off mid-use. If your system draws far less, the PSU runs inefficiently and generates unnecessary heat.

To find the right wattage, start with your CPU and graphics card, because they consume the most power. Look up the TDP (thermal design power) for each. A typical modern CPU has a TDP of 65 to 125 watts. A mid-range graphics card might be 200 to 250 watts. A high-end card can be 350 watts or more. Add those two numbers, then add 100 watts for the motherboard, storage, fans, and other components. That total is your target wattage.

For example: a CPU with a 95-watt TDP plus a graphics card with a 220-watt TDP equals 315 watts. Add 100 for the rest of the system, and you need a PSU rated for at least 415 watts. In practice, you would buy a 500 or 550-watt unit, which gives you headroom and lets the PSU run at a comfortable load rather than at its limit.

If you do not have a graphics card, or you are using the integrated graphics built into your CPU, subtract 200 watts from the calculation. A basic office computer with no dedicated graphics card might need only 300 to 400 watts total.

Checking your current PSU's wattage and condition

Before you buy a replacement, confirm that the PSU is actually the problem. If your computer will not turn on, the PSU is one of several possible culprits — the motherboard, power button, or a loose cable could also be at fault.

Look at the label on the side of your current PSU. It lists the wattage (usually in large text), the voltage rails, and the maximum amperage on each rail. Write down the wattage number. If the label is faded or missing, you can usually find the model number on the label and search for the specs online.

If your computer is turning on but shutting down unexpectedly under load — especially when you are gaming or running video editing software — the PSU may be failing even if it still powers on. A failing PSU often cannot deliver its rated wattage anymore, so the system loses power when demand spikes. This is harder to diagnose than a dead PSU, but the fix is the same: replace it.

Buying a replacement PSU with the right connector type

PSUs come in different physical sizes and connector types. The most common size is ATX, which fits in most desktop cases. Smaller cases use SFX or TFX units. Check your case manual or look inside to see which size you have — the old PSU will tell you.

The connector type matters more. Most modern PSUs use a 24-pin connector for the motherboard and an 8-pin or 4-pin connector for the CPU. Graphics cards use 6-pin or 8-pin connectors. Storage drives use SATA connectors. If your motherboard or graphics card has a specific connector type, your new PSU must have it too.

When you shop for a replacement, match the wattage you calculated, the physical size of your old unit, and the connector types your components need. Most retailers list these specs clearly. If you are unsure, bring a photo of your old PSU's label and your motherboard manual to the store, or post the photos in a computer hardware forum and ask for confirmation before you buy.

Preparing your workspace and gathering tools

You need very little to replace a PSU. Gather a Phillips-head screwdriver (the most common type), a flashlight or headlamp so you can see inside the case, and a small container to hold screws so you do not lose them. Some people also keep a grounding strap handy to avoid static discharge, though it is not strictly necessary if you touch the metal case frame frequently while working.

Set your computer on a flat, clean surface with good lighting. Make sure you have room to open the side panel and reach inside. Unplug the power cable from the wall outlet — do not just turn the computer off. A PSU can hold a charge even when the computer is off, and unplugging it fully removes that risk.

Before you open the case, take a photo of the cable layout inside. Use your phone camera and get a clear shot of where each cable connects. This photo is your reference when you reconnect everything. If you have a complex system with many cables, take multiple photos from different angles.

Disconnecting the old PSU step by step

Open the side panel of your case. Locate the PSU, which is usually in the bottom rear corner. You will see cables running from it to the motherboard, graphics card, storage drives, and fans.

Start by disconnecting the 24-pin motherboard connector. This is the largest connector and usually sits near the top of the motherboard. Gently pull the connector straight out — do not twist or wiggle it side to side. If it is stuck, use a small flathead screwdriver to lift the small tab on the connector while you pull.

Next, disconnect the 8-pin or 4-pin CPU power connector, which is usually near the top of the motherboard close to the CPU. Then disconnect any 6-pin or 8-pin connectors going to your graphics card. These connectors are smaller and have a tab you can press to release them.

Disconnect the SATA power connectors from your storage drives and any other SATA devices. These are flat, L-shaped connectors that slide straight out. Finally, disconnect any fan connectors if your case fans are powered by the PSU rather than the motherboard.

Once all the cables are disconnected, locate the mounting bolts holding the PSU to the case. There are usually four bolts on the back of the case, visible from outside. Remove them with your screwdriver and keep them in your small container. Gently slide the PSU out of the case toward the front or side, depending on your case design.

Installing the new PSU and reconnecting cables

Slide the new PSU into the case in the same orientation as the old one. The fan usually faces downward if your case has ventilation holes in the bottom, or toward the rear if it does not. Align the mounting holes and insert the four bolts. Tighten them snugly but do not over-tighten — you want them firm enough that the PSU does not move, not so tight that you strip the threads.

Now reconnect the cables in reverse order. Start with the 24-pin motherboard connector. Line up the connector with the socket on the motherboard — the keyed shape means it only fits one way — and push it in firmly until you hear or feel a click. The connector should sit flush against the motherboard.

Reconnect the 8-pin or 4-pin CPU power connector next, then the graphics card connectors if you have a dedicated GPU. Reconnect the SATA power connectors to your storage drives, and any fan connectors. Use your reference photo to confirm each connection matches the original layout.

Check that all connectors are fully seated. A loose connector is the most common reason a newly replaced PSU does not work. If a connector feels loose, disconnect it and reconnect it — sometimes the first attempt does not seat fully.

Testing the new PSU and troubleshooting

Close the case and plug the power cable back into the wall outlet. Press the power button on your computer. It should turn on normally. Listen for the fan in the PSU to spin up — you should hear a brief whirring sound as the system boots.

Let the computer run for a few minutes and watch for any unusual behavior. The screen should display normally, and you should not see any error messages. If the computer turns on and runs without issues, the replacement was successful.

If the computer does not turn on, unplug it when ready and check your connections. The most common cause is a loose 24-pin motherboard connector or a 4-pin CPU connector that did not seat fully. Disconnect and reconnect each one, then try again.

If the computer turns on but when ready shuts down, the PSU may be detecting a short circuit or overload. This usually means a connector is plugged into the wrong socket. Check your reference photo and verify that each connector is in the correct location. If you cannot find the problem, unplug the computer and ask for help in a computer hardware forum — describe what happened and include photos of your cable layout.

Frequently Asked Questions

Can I replace a PSU while the computer is on?

No. Always unplug the power cable from the wall outlet before you open the case or touch any internal components. A PSU can hold an electrical charge even when the computer is off, and touching live components can damage your hardware or injure you.

What if my new PSU has cables I do not need?

Most PSUs come with more cables than you will use. You can leave the extra cables coiled inside the case or tie them back with velcro straps to keep them out of the way. Do not cut them off — you might need them if you add components later.

Do I need to update my BIOS or drivers after replacing the PSU?

No. The PSU is a power delivery component, not a data component. Your BIOS and drivers do not interact with it directly. Once the PSU is installed and the computer turns on, everything should work as before.

How do I know if my PSU is actually dead or just failing?

A dead PSU will not turn the computer on at all. A failing PSU usually lets the computer start but shuts it down under heavy load — when you are gaming, editing video, or running other demanding software. If your computer runs fine during light use but cuts off during intensive tasks, the PSU is likely failing and should be replaced.

What is 80 Plus certification and does it matter?

80 Plus is a standard that measures how efficiently a PSU converts wall power into usable power. A PSU with 80 Plus Bronze certification converts at least 80 percent of the power it draws. Higher tiers (Silver, Gold, Platinum) are more efficient. A more efficient PSU runs cooler, lasts longer, and costs less to operate over time, though it usually costs more upfront.