What a power supply does and why it fails

A power supply (also called a PSU, or power supply unit) converts electricity from your wall outlet into the right voltage and current for each part of your computer — motherboard, graphics card, storage drives, fans. When it fails, your computer won't start, or it starts and shuts down randomly, or components stop working one at a time.

Power supplies fail because their internal capacitors wear out over time, usually after five to ten years of use. They can also fail suddenly if there's a power surge, if dust clogs the cooling fan inside, or if the unit was undersized for your components from the start. Unlike some computer parts, a failed power supply cannot be repaired — you replace it.

Before you buy a replacement, you need to know three things: the wattage of your current supply, the physical size (called the form factor), and whether your computer's case will accept a standard replacement. This article walks you through identifying what you have, choosing the right replacement, and installing it safely.

Key Takeaways

  • Your current power supply's wattage and form factor are printed on a label inside your computer case; write these down before shopping for a replacement.
  • A replacement power supply should match or exceed your current wattage and fit the same form factor (usually ATX, Micro-ATX, or SFX).
  • Replacing a power supply requires unplugging the computer, opening the case, disconnecting four or five cable groups, removing four bolts, and reversing the steps with a new unit.
  • The job takes 20 to 40 minutes and requires only a Phillips screwdriver; if you are not comfortable opening your case or working inside it, a computer repair shop can do this for $50 to $150 in labor.
  • After installation, test the computer by powering it on and checking that all fans spin and the system stays on for at least a few minutes.

Finding your current power supply's wattage and size

The easiest way is to look at the label on the power supply itself. Shut down your computer completely, unplug it from the wall, and wait a few minutes. Open your case — most desktop cases have a side panel held by two or four thumbscrews or Phillips screws. The power supply is usually a metal box in one of the bottom corners, often with a fan visible on the back.

On the side or back of the power supply, you will see a white or yellow label listing the wattage (for example, 650W, 750W, 1000W). Write this number down. The form factor — the physical size and shape — is usually printed on the same label or is obvious from the shape. Standard desktop supplies are ATX (about 5.9 inches wide, 3.4 inches tall, 5.5 inches deep). Smaller cases use Micro-ATX or SFX supplies, which are noticeably more compact.

If the label is worn or missing, you can estimate the wattage by looking at your motherboard manual or by searching your computer's model number online. You can also open your case and count the connectors: a 500W supply typically has one 8-pin CPU connector and one 6-pin or 8-pin PCIe connector; a 750W or higher usually has two 8-pin PCIe connectors. This is not exact, but it gives you a ballpark figure.

Choosing the right replacement power supply

Match the wattage and form factor of your current supply, or go slightly higher. If your current supply is 650W, a 750W replacement will work fine and gives you a small safety margin. Do not go significantly lower — a power supply that is too weak for your components will fail quickly or cause random shutdowns.

Buy from a reputable manufacturer. Seasonic, Corsair, EVGA, and Thermaltake make reliable supplies at a range of prices. Avoid no-name brands from online marketplaces, as they often overstate their wattage and fail within months. A quality replacement typically costs $60 to $150 depending on wattage.

Check the cable configuration before you buy. Your current supply has a specific set of connectors — a 24-pin motherboard connector, an 8-pin CPU connector, and one or more PCIe connectors for graphics cards. A new supply will have the same connectors, but some newer supplies use modular cables (you plug in only the cables you need) while older supplies have all cables permanently attached. Either type works; modular supplies are tidier but cost more.

Preparing to open your case and disconnect the old supply

Shut down your computer and unplug it from the wall outlet. Wait at least two minutes — this allows any residual charge in the power supply to drain. Touch a metal part of the case to discharge any static electricity from your body, or wear an anti-static wrist strap clipped to the case.

Open the side panel of your case. You will see the power supply mounted in a corner, usually the bottom rear. Before you disconnect anything, take a photo of the cable connections with your phone. This gives you a reference when you reconnect the new supply.

The power supply has four main cable groups: the 24-pin motherboard connector, the 8-pin (or 4-pin) CPU connector near the top of the motherboard, the 6-pin or 8-pin PCIe connectors going to the graphics card, and the SATA or Molex connectors going to storage drives and fans. Some supplies also have a separate 4-pin connector for case fans. Gently pull each connector straight out of its socket — do not twist or yank. If a connector is stuck, wiggle it gently side to side while pulling.

Removing the old power supply and installing the new one

Once all cables are disconnected, locate the four bolts holding the power supply to the case. They are usually on the back of the case, visible from outside. Use a Phillips screwdriver to remove all four bolts. Slide the power supply out of the case toward the front — it is heavy, so support it with both hands.

Set the new power supply in the same position, with the fan facing down (or toward the case opening, depending on your case design). Align the bolt holes and insert all four bolts by hand first, then tighten them with the screwdriver. Do not over-tighten — snug is enough.

Reconnect the cables in reverse order. Start with the 24-pin motherboard connector — it is keyed so it only fits one way, but push firmly until it clicks. Next, connect the 8-pin CPU connector near the top of the motherboard. Then connect the PCIe connectors to your graphics card (if you have one), and finally the SATA or Molex connectors to drives and fans. Use your photo as a reference. Each connector should click or seat firmly when fully inserted.

Testing the new power supply

Before you close the case, plug the power cable back into the wall outlet and flip the power switch on the back of the power supply to the ON position. Press the power button on the front of your computer. The system should start normally — you will hear the fans spin up and see lights on the motherboard and graphics card.

Let the computer run for at least five minutes. Listen for any unusual sounds, and check that all fans are spinning. If the computer starts and runs normally, shut it down, close the case, and you are done. If the computer does not start, or starts and when ready shuts down, turn off the power supply, unplug it, and check that all connectors are fully seated. Reseat any that feel loose and try again.

If the computer still does not start after you have checked all connections, the new power supply may be defective. Contact the manufacturer or retailer for a replacement under warranty. Do not attempt to repair or open the power supply itself — it contains high-voltage capacitors that can cause serious injury even when unplugged.

When to call a professional instead

If you are not comfortable opening your computer case, working with small connectors, or handling the power supply itself, a local computer repair shop can replace it for you. Labor typically costs $50 to $150 depending on the shop and your location. You can either buy the power supply yourself and bring it to the shop, or let them source it for you (they will usually charge a markup on the part).

You should also call a professional if your computer does not start after you install the new supply and you have checked all connections. There may be a secondary issue — a failed motherboard, a damaged cable, or a short circuit — that requires diagnosis beyond a straightforward replacement.

Frequently Asked Questions

Can I use a power supply with higher wattage than my current one?

Yes. A 750W supply will work fine in a computer that came with a 650W supply. The wattage is the maximum the supply can deliver; your computer will only draw what it needs. Higher wattage gives you headroom if you upgrade components later.

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

If you bought a modular supply, you straightforward don't plug in the unused cables. If you bought a non-modular supply with extra cables, coil them neatly and find them with velcro straps or zip ties so they don't block airflow inside the case.

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

No. A power supply is a passive component — it has no firmware or drivers. Once it is physically installed and connected, it works when ready.

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

If your computer won't start at all, check that the power cable is plugged in and the outlet works (plug in a lamp to test). If the outlet is fine, flip the power switch on the back of the power supply to OFF, wait 10 seconds, and flip it back to ON. If the computer still won't start, the power supply is likely dead. A repair shop can test it with a power supply tester to confirm.

Is it safe to keep using a power supply that randomly shuts down my computer?

No. A failing power supply can damage your motherboard, graphics card, or storage drives if it delivers unstable voltage. If your computer shuts down randomly and a power supply replacement is the likely cause, replace it as soon as you can to avoid losing data or damaging other components.