When and why to replace your power supply

A power supply fails when it can no longer deliver stable electricity to your components. The most common signs are a computer that won't turn on at all, random shutdowns during use, or a burning smell from inside the case. If your system is five to ten years old and starts shutting down under load — like during gaming or video editing — the power supply is often the culprit.

You might also replace a power supply to upgrade to more powerful components. If you're adding a graphics card that draws more power than your current supply can handle, or switching from a 400-watt supply to a 750-watt one, you'll need to physically swap the unit. A failing supply can damage other parts, so replacing it quickly prevents a cascade of failures that would cost far more.

Key Takeaways

  • A power supply typically lasts five to ten years; signs of failure include random shutdowns, the computer not turning on, or a burning smell.
  • You need to know your case size (ATX, Micro-ATX, or Mini-ITX) and the wattage your components actually require before buying a replacement.
  • The replacement process involves shutting down completely, unplugging from the wall, removing the old supply, and installing the new one — no special tools required beyond a screwdriver.
  • All cables plug into the same connectors on modern supplies, so you can reuse your existing cables or use the new ones that come with the replacement.

Choosing the right power supply for your case and components

Your case size determines which power supply will physically fit. Most desktop computers use a standard ATX power supply, which is a rectangular box about 5 inches wide, 3 inches tall, and 6 inches deep. Smaller cases use Micro-ATX supplies (slightly shorter) or Mini-ITX supplies (much smaller and less common). Open your case and measure the power supply, or check your case manual — it will state which size it accepts.

Wattage is the second decision. Your power supply must deliver enough power for your CPU, graphics card, storage drives, and motherboard combined. A basic office computer with no graphics card needs 300 to 400 watts. A gaming system with a mid-range graphics card needs 550 to 650 watts. A high-end gaming or workstation build with a top-tier graphics card needs 750 to 1000 watts. If you're unsure, add 100 to 150 watts to what you think you need — an oversized supply runs cooler and lasts longer than one running at its limit.

Buy from a manufacturer with a track record: Corsair, EVGA, Seasonic, and Thermaltake are common choices. Avoid the cheapest no-name supplies; a failed power supply can take your motherboard or graphics card with it. Most quality supplies come with a five to ten year warranty.

What you need before you start

Gather a Phillips head screwdriver — that's the only tool you need. Have a flashlight or headlamp handy if your workspace is dim. Take a photo of your current power supply's cable connections with your phone before you unplug anything; this makes it much faster to reconnect everything correctly on the new supply.

If you're keeping your old cables, label them with tape and a marker as you disconnect them. Modern power supplies use the same connector types, so old cables work with new supplies, but labeling saves time and prevents plugging a cable into the wrong port. If your new supply comes with cables, use those instead — they're may provide to be correct.

Removing the old power supply

Shut down your computer completely and unplug the power cable from the wall outlet. This is the most important step — working inside a powered-off, unplugged case is safe; working inside a plugged-in case can cause serious injury. Wait 30 seconds for any residual charge to drain from the power supply.

Open your case by removing the side panel. Most cases have two thumbscrews or regular screws on the back; remove them and slide the panel off. Locate the power supply — it's usually in the bottom rear corner of the case, a large metal box with cables running from it to your motherboard, graphics card, and storage drives.

Disconnect every cable from the power supply. Start with the large 24-pin connector to the motherboard, then the smaller 4-pin or 8-pin connector near the CPU. Unplug any cables going to your graphics card (usually two 6-pin or 8-pin connectors). Disconnect the SATA power cables to your hard drives or SSDs, and any other connectors you see. Take your time — forcing a connector can break it.

Remove the four screws holding the power supply to the case. These are on the back of the case, around the power supply's mounting bracket. Keep these screws in a small container so you don't lose them. Carefully slide the power supply out of the case toward the front or side, depending on your case design.

Installing the new power supply

Slide the new power supply into the same position, with the fan facing down if your case has ventilation holes in the bottom, or facing up if it doesn't. The mounting holes should align with the case. Insert the four screws and tighten them firmly but not so hard that you strip the threads — hand-tight plus a quarter turn is usually right.

Reconnect the cables in reverse order. Start with the 24-pin motherboard connector — it only fits one way, with a small clip on the connector that locks into place. Plug in the 4-pin or 8-pin CPU power connector near the top of the motherboard. Reconnect any graphics card power cables. Plug in SATA power cables to your storage drives. Double-check that every connector is fully seated — a loose connection can cause random shutdowns or failure to power on.

Close your case and plug the power cable back into the wall outlet. Press the power button on the front of your case. The computer should start normally. If it doesn't, unplug when ready, open the case, and check that all connectors are fully inserted.

Testing and troubleshooting

Let your computer run for at least 15 minutes after the replacement. Listen for any unusual fan noise from the power supply itself — a new supply should be nearly silent or have a quiet, steady hum. If you hear grinding, clicking, or loud whining, unplug when ready and contact the manufacturer about a replacement.

Check that your computer stays on during normal use. Open a web browser, watch a video, or run a program you use regularly. If the computer shuts down unexpectedly, unplug and check that all power connectors are fully seated. If it still shuts down after reseating the cables, the new supply may be defective — contact the retailer or manufacturer.

If your computer won't turn on at all after the replacement, the most common cause is a loose connector. Unplug, open the case, and reseat the 24-pin motherboard connector and the CPU power connector. Make sure the power supply's own power switch (usually on the back) is set to the ON position. If it still won't start, try the old power supply temporarily to confirm the problem was actually the supply and not something else.

When to replace cables

You can reuse your old cables with a new power supply if they're in good condition — no fraying, burns, or bent pins. Most modern supplies use the same connector types, so compatibility is not an issue. However, if your old cables show any damage, or if your new supply comes with cables and you want a fresh start, use the new ones instead.

Some high-end supplies come with modular cables, meaning you only connect the cables you actually need. This reduces clutter inside your case and improves airflow. If you're upgrading to a modular supply, take time to plan which cables you need before you plug them in — the manual will show you which ports are which.

Frequently Asked Questions

Can I replace a power supply while the computer is on?

No. Always shut down completely and unplug from the wall. Working inside a powered case risks serious electrical injury. Even after unplugging, wait 30 seconds for the power supply to discharge before touching internal components.

What if my new power supply has extra cables I don't need?

Leave them disconnected and coiled neatly inside the case, or bundle them with velcro straps and tuck them out of the way. Extra cables don't hurt anything if they're not plugged in. Don't cut them off — you might need them if you upgrade components later.

How do I know if my power supply is actually failing or if something else is wrong?

If the computer won't turn on at all, try a different power cable first — the cable itself can fail. If it still won't start, the supply is likely bad. If the computer shuts down randomly under load, try reseating all power connectors; if that doesn't help, the supply is probably failing. A burning smell almost always means the supply has failed and should not be used.

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

No. A power supply has no drivers or software. Once it's physically installed and all cables are connected, your computer will recognize it automatically and work normally.

Is it safe to use a power supply that's much more powerful than I need?

Yes. An oversized supply is actually safer — it runs cooler because it's not working at maximum capacity, which extends its lifespan. The only downside is cost; you're paying for wattage you don't use. A supply sized 100 to 150 watts above your actual need is the practical sweet spot.