When and why you need a new power supply

Your power supply converts wall electricity into the right voltage for each component in your computer. When it fails, your PC won't turn on at all, or it shuts down randomly under load. A failing power supply can also damage other parts — if it sends unstable power to your motherboard or graphics card, you may lose them too.

Most power supplies last five to ten years. You might need to replace yours if your PC won't start, restarts without warning, makes a loud humming or grinding noise from inside the case, or smells like burning plastic. Before you assume the power supply is dead, check that the wall outlet works (plug a lamp into it) and that the power cable isn't loose where it connects to the back of the case.

Replacing a power supply is one of the simpler hardware repairs. You do not need special tools beyond a Phillips screwdriver, and the whole job usually takes 20 to 30 minutes. The hardest part is choosing the right replacement — power supplies come in different sizes and power ratings, and picking the wrong one can leave you unable to power all your components.

Key Takeaways

  • Power supplies are rated in watts, and your replacement must meet or exceed the total power draw of your CPU, graphics card, and other components combined.
  • Most desktop PCs use an ATX power supply, but some smaller cases use SFX or TFX models, so measure your current supply or check your case manual before ordering.
  • Unplug the PC from the wall outlet and flip the power supply switch to OFF before you open the case, even if the PC is already off.
  • All cables plug into connectors that only fit one way, so you cannot install them backwards — push each connector until you hear or feel a click.
  • If your new power supply does not come with all the cables you need, you can buy extension cables or modular cables separately, but never splice or modify power cables yourself.

Finding the right wattage and size for your replacement

Power supplies are measured in watts. A typical office PC needs 300 to 500 watts. A gaming PC with a high-end graphics card might need 750 watts or more. If you buy a supply that is too weak, your PC will shut down when you run demanding software. If you buy one that is too strong, you waste money, but the PC will still work fine.

The safest approach is to add up the power requirements of your major components. Your CPU and graphics card use the most power — you can find their wattage in the spec sheet on the manufacturer's website. Add 100 to 150 watts for everything else (motherboard, storage drives, fans, lights). If your CPU needs 105 watts and your graphics card needs 320 watts, you need at least 525 watts, so a 650-watt supply gives you headroom.

If you do not know what components you have, open your PC case and look at the stickers on the graphics card and CPU cooler, or use a tool like CPU-Z (free software that runs on Windows) to see what is installed. If you cannot find the specs, a 750-watt supply is a safe middle ground for most gaming PCs, and a 500-watt supply works for most office machines.

The physical size matters too. Most desktop cases use an ATX power supply, which is a rectangular box about 5.5 inches wide, 3.4 inches tall, and 5.5 inches deep. Some smaller cases use SFX supplies (about 4.9 by 3.9 by 4.3 inches) or TFX supplies (even smaller). Check your case manual or measure your current power supply before you order. Ordering the wrong size means the new supply will not fit in your case.

Preparing your PC and gathering what you need

Shut down your PC completely and unplug the power cable from the wall outlet. This is the most important safety step — even when your PC is off, the power supply still has electricity flowing through it as long as it is plugged in. Flipping the power switch on the back of the power supply to OFF adds another layer of safety, but unplugging is what actually cuts the power.

Let the PC sit for a few minutes. The power supply has capacitors inside that store electrical charge, and waiting gives them time to discharge. This is especially important if the PC was running before you shut it down.

Gather a Phillips screwdriver (the cross-shaped one, not a flat-head). You do not need an anti-static wrist strap for this job, but if you have one, wear it clipped to a metal part of the case. If you do not have one, touch a metal part of the case occasionally to discharge any static electricity from your body. Have your new power supply and the old one nearby so you can compare the connectors as you work.

Removing the old power supply

Open your PC case. Most cases have two side panels held on with four to six screws each. Remove the screws and slide or lift the panels off. You are looking for the power supply, which is usually a metal box in one of the bottom corners of the case, often with a fan visible on the back.

Before you touch anything, take a photo of how the cables are connected. This helps you remember where everything goes when you install the new supply. Pay special attention to the large 24-pin connector that plugs into the motherboard, and the smaller 8-pin or 4-pin connector near the CPU.

Unplug every cable from the power supply. Start with the motherboard connectors (the large one and the small one near the CPU), then unplug the cables going to the graphics card, storage drives, and fans. Some cables may be held in place with clips or ties — gently pull these away before you unplug. Do not force anything; if a connector feels stuck, wiggle it gently side to side while you pull.

Locate the mounting screws that hold the power supply to the case. These are usually on the back of the case, around the power supply fan. Remove all of them — usually four screws. Slide the power supply toward the front of the case (away from the motherboard) and lift it out. It is heavier than it looks, so use both hands.

Installing the new power supply

Slide the new power supply into the same spot, with the fan facing down (or in the direction the old one faced). The mounting holes should line up. If they do not, the power supply is the wrong size — do not force it. Insert the four mounting screws and tighten them snugly, but do not over-tighten; you should not need to strain.

Plug the large 24-pin motherboard connector into the motherboard. This connector is keyed — it only fits one way — so line it up and push firmly until you hear a click. Plug the smaller 4-pin or 8-pin CPU power connector into the socket near the CPU cooler (usually at the top of the motherboard). Again, push until you hear a click.

Plug the graphics card power connectors (usually 6-pin or 8-pin) into the graphics card. If your graphics card has two connectors, both must be plugged in. Plug the SATA power connectors (the flat ones with three rows of pins) into your storage drives — solid-state drives (SSDs) and hard drives both use these. Plug the Molex connectors (the larger rectangular ones) into any fans that need power, or into older hard drives if you have them.

If your new power supply does not have enough cables, you have two options. You can buy extension cables (which plug onto the existing cables and add length) or modular cables (which replace the built-in cables entirely). Never cut, splice, or modify power cables yourself — this creates a fire and shock hazard. If you cannot find the right cable, contact the power supply manufacturer for help.

Testing and closing up

Before you close the case, flip the power supply switch to ON and plug the power cable back into the wall outlet. Press the power button on your PC. It should start normally. If nothing happens, unplug when ready and check that all connectors are fully seated — push each one in again until you hear a click.

Once the PC starts, let it run for a minute or two. Listen for any unusual noises. The power supply fan should spin quietly; a loud grinding or squealing sound means something is wrong. If the PC starts and runs quietly, shut it down normally (do not just unplug it), unplug the power cable, and close the case.

Reinstall the side panels and screw them back in. You are done. Your new power supply should work reliably for five to ten years, just like the old one did.

Troubleshooting if the PC won't start

If you plug everything in and the PC still will not start, the most common cause is a loose connector. Unplug the power cable from the wall, flip the power supply switch to OFF, wait 30 seconds, then check every single connector. Push each one in firmly until you hear a click. Pay special attention to the 24-pin motherboard connector and the CPU power connector — these are the most likely to be loose.

If the PC still will not start, check that you plugged the motherboard power connector into the right socket. It should go into the large rectangular socket on the edge of the motherboard, usually near the right side. If you plugged it into a different socket by mistake, unplug it and move it to the correct one.

If the PC still will not start after checking all connectors, the new power supply may be defective. Contact the manufacturer for a replacement. Do not try to repair a power supply yourself — they contain high-voltage components that can cause serious injury even when unplugged.

Frequently Asked Questions

Can I use a power supply from a different brand than my old one?

Yes. Power supplies from different manufacturers all use the same connector types and fit the same case sizes. The only thing that matters is the wattage and the physical size (ATX, SFX, or TFX). Brand does not affect compatibility.

What if my case has no room for the power supply I need?

If your case is too small for an ATX supply, you can buy an SFX or TFX supply instead, as long as your components do not need more power than a smaller supply can provide. If even the smallest supply is too weak for your graphics card and CPU, you may need a larger case. Check the case manual for the maximum power supply size it supports.

Do I need to reinstall Windows or my programs after replacing the power supply?

No. The power supply does not store any data or software. Once you install it and the PC starts, everything will be exactly as it was before. Your programs, files, and operating system are on your storage drive, not the power supply.

Is it safe to touch the inside of the PC while the power supply is plugged in but the PC is off?

No. Always unplug the power cable from the wall outlet before you open the case, even if the PC is off. The power supply has electricity flowing through it as long as it is plugged in, and touching the wrong part could cause a shock.

What should I do with my old power supply?

Take it to an electronics recycling center. Power supplies contain materials that are harmful to the environment if thrown in the trash. Most cities have a recycling program for electronics — search online for "electronics recycling near me" to find a location.