A power supply replacement is a straightforward job you can do yourself in about 30 minutes

Your PC's power supply converts wall electricity into the right voltage for your motherboard, graphics card, and drives. When it fails, nothing in your computer will turn on—or it may shut down randomly under load. Replacing it means unplugging the old unit, unscrewing it from the case, disconnecting its cables, sliding in a new one, reconnecting those same cables, and screwing it back in. You do not need special tools or technical training. The hardest part is choosing the right replacement.

The entire process takes about 30 minutes if you work carefully and take photos as you go. Most of that time is spent double-checking connections to make sure nothing is loose. If you have never opened your PC before, this is a good first project—the steps are straightforward, the risk of damage is low, and the payoff is when ready.

Key Takeaways

  • Your replacement power supply must have enough wattage for your system; a good rule is to add 30 percent to your current system's peak draw and buy that wattage or higher.
  • The physical size of your power supply matters—ATX is the standard for full-size cases, but smaller cases use SFX or TFX units that cost more and are harder to find.
  • Unplug the PC from the wall, flip the power supply's rear switch to OFF, and wait five minutes before touching anything inside to discharge static electricity safely.
  • Take a photo of each cable connection before you disconnect it, so you know where everything plugs back in.
  • Buying a unit from a known brand with a warranty (Corsair, EVGA, Seasonic, or similar) costs a little more but saves you from a second failure in two years.

Finding the right wattage for your system

The wattage you need depends on what components are inside your PC. A system with a mid-range processor and no graphics card might draw 300 watts under full load. Add a high-end graphics card and you could be at 650 watts or more. The safest approach is to add 30 percent to your system's peak draw—so if you estimate 500 watts, buy a 650-watt unit.

If you do not know your system's power draw, look up your processor and graphics card on their maker's website and note the "TDP" (thermal design power) for each. Add those numbers together, then add 100 watts for the motherboard, drives, and fans. That gives you a rough estimate. You can also use an online power calculator: search "PC power supply calculator" and enter your components. The result will tell you a safe wattage to buy.

Buying more wattage than you need is not wasteful—a 750-watt supply running a 500-watt system is more efficient and runs cooler than a 500-watt supply at its limit. Buying less wattage than you need will cause random shutdowns or prevent the PC from starting at all.

Checking the physical size and connector type

Power supplies come in three main sizes. ATX is the standard—it is 150 mm wide, 86 mm tall, and 140 mm deep, and it fits in almost every full-size PC case. SFX is smaller (125 × 100 × 63 mm) and designed for compact cases; it costs more and has fewer models to choose from. TFX is even smaller and rarer. Check your case's manual or measure the space where your current power supply sits to know which size you need.

Also check what connectors your motherboard and graphics card use. Most modern systems use a 24-pin connector for the motherboard and an 8-pin or 6-pin connector for the graphics card. Older systems may use a 20-pin motherboard connector. When you buy a replacement, the product page will list what connectors it includes. Make sure it has the ones you need. Mismatched connectors mean you cannot complete the installation, so verify this before you order.

Preparing your PC for the swap

Shut down your PC completely and unplug the power cable from the wall. Do not just turn it off—physically remove the plug. Flip the power supply's rear switch to OFF (the small toggle switch on the back of the unit). Wait five minutes. This discharges the capacitors inside the power supply, which can hold a charge even when unplugged and can give you a painful shock.

Open your PC case. Most cases have two thumbscrews or latches on the side panel; loosen them and slide the panel off. You will now see the power supply mounted in the rear corner of the case, usually with a large fan facing down or toward the case interior. Before you touch anything, take a photo of the power supply's cable connections with your phone. You need to see where the 24-pin motherboard cable, the 8-pin CPU cable, the graphics card cables, and any SATA or Molex drive cables are plugged in. This photo is your map for reconnecting everything.

Removing the old power supply

Unplug every cable from the power supply. Start with the large 24-pin connector on the motherboard—grip the connector itself, not the wires, and pull straight out. Then unplug the 8-pin CPU power cable above the processor. Unplug any 6-pin or 8-pin graphics card connectors. Finally, unplug any SATA power connectors (the flat L-shaped ones) or Molex connectors (the larger rectangular ones) that go to drives or fans.

Once all cables are free, locate the four screws on the rear of the case that hold the power supply in place. They are usually on the outside of the case, visible from behind. Unscrew all four. Gently slide the power supply toward the front of the case and lift it out. It is heavy—use both hands. Set it aside in a safe place; you may want to keep it in case you need to return the new unit.

Installing the new power supply

Slide the new power supply into the rear corner of the case, with the fan facing the same direction as the old one (usually downward). Align the four screw holes and insert the four screws. Tighten them snugly but do not over-tighten; you want them firm, not stripped.

Now reconnect the cables using your photo as a guide. Start with the 24-pin motherboard connector—align the notch on the connector with the notch on the motherboard socket and push straight in until it clicks. Plug in the 8-pin CPU power cable above the processor. Reconnect any graphics card power cables. Finally, plug in SATA or Molex connectors to any drives or case fans that need them. Double-check every connection. The connectors should click or seat firmly; if one feels loose, unplug it and try again. A loose connection can cause the PC to shut down or fail to start.

Testing the new power supply

Close your PC case and plug the power cable back into the wall. Flip the power supply's rear switch to ON. Press the power button on your PC. It should start normally. Listen for the fans to spin up and watch for the motherboard lights to come on. If the PC starts and stays on for a few minutes, the replacement was successful.

If the PC does not start, unplug it when ready and check every cable connection against your photo. The most common mistake is a loose 24-pin motherboard connector or a graphics card power cable that did not seat all the way. Reseat each one and try again. If it still does not start after checking all connections, the new power supply may be defective—contact the seller or manufacturer for a replacement under warranty.

Choosing a reliable power supply brand

Not all power supplies are built the same. Cheap units from unknown brands may fail after a year and can damage your motherboard or graphics card on the way out. Spending a little more on a unit from Corsair, EVGA, Seasonic, Thermaltake, or be quiet! gives you a warranty (usually three to ten years) and a much lower failure rate.

Look at the product reviews on the retailer's website and on independent review sites. A power supply with hundreds of five-star reviews is a safer bet than one with ten reviews and mixed ratings. Also check the warranty length—a five-year or ten-year warranty is a sign the maker stands behind the product. The extra cost upfront saves you money and frustration later.

Frequently Asked Questions

Can I reuse the cables from my old power supply?

No. Power supply cables are not universal. Even if the connectors look the same, the pinout (which wire does what) can differ between models and brands. Using the wrong cable can damage your motherboard or graphics card. Always use the cables that come with your new power supply.

What if my case does not have a power supply fan vent?

Older cases sometimes have the power supply mounted with the fan facing the case interior instead of venting outside. This is fine—the fan will pull air from inside the case and exhaust it out the back. Just make sure the power supply has room to breathe and is not blocked by cables or drives.

Do I need to install drivers for a new power supply?

No. A power supply has no drivers or software. Once it is plugged in and the PC starts, it works. Your operating system does not need to recognize it or install anything.

How do I know if my power supply is actually dead?

If your PC will not turn on at all and the motherboard lights do not come on, the power supply is usually the culprit. You can also check by flipping the rear switch to ON and listening for the fan to spin—a working power supply's fan will turn on even if nothing is plugged into it. If the fan does not spin and the rear switch is ON, the power supply is dead.

Is it safe to touch the inside of my PC while the power supply is off?

Yes, as long as you have unplugged the PC from the wall and waited five minutes. After that wait, the power supply is discharged and safe to touch. To be extra safe, touch a metal part of the case before you start work to ground yourself and discharge any static electricity from your body.