Where the power supply sits in your case
Your power supply is a metal box, usually the size of a thick paperback book, mounted in one of the bottom corners of your PC case. In most desktop towers, it sits in the lower rear corner — either bottom-left or bottom-right depending on the case design. The fan on the power supply typically faces downward or toward the back of the case to pull in cool air.
If you open the side panel of your case, look for a large rectangular metal box with a thick cable bundle coming out of it. That cable bundle connects to your motherboard, graphics card, and storage drives. The power supply is the only component that has its own separate power cord running to the wall outlet.
Key Takeaways
- The power supply is a metal box in the lower rear corner of your case, with a fan and a thick bundle of cables attached to it.
- You can identify it by the single thick power cord that runs from it to your wall outlet — no other component has this.
- The wattage rating is printed on a label on the side or back of the unit, usually in the 400–1200 watt range for consumer PCs.
- The fan on the power supply should spin freely; if it does not move at all, the unit may have failed.
- Do not open the power supply itself — the capacitors inside hold a charge even when unplugged and can cause serious injury.
Reading the label to find wattage and model
The power supply has a label stuck to one of its sides or the back. This label shows the wattage (usually 400W, 500W, 650W, 750W, or higher), the brand name, and the model number. The wattage tells you how much power the unit can deliver to your components — a higher number means it can handle more demanding hardware.
Write down the wattage and model number if you need to replace the unit or check whether it is strong enough for an upgrade. The label also lists the voltage and amperage for each rail (12V, 5V, 3.3V), but for most purposes you only need the total wattage in watts.
Checking whether the power supply is working
A working power supply has a fan that spins when the PC is on. If you turn on your PC and the fan does not move, or if the PC does not turn on at all, the power supply may have failed. You can also listen for a faint hum from the unit when the system is running — this is normal.
If the PC turns on but shuts down unexpectedly, or if components are not getting power, the power supply may be failing even if the fan spins. In these cases, the unit usually needs to be replaced. Power supplies do not repair well and are cheaper to swap out than to troubleshoot.
How to safely access the power supply
To see the power supply clearly, shut down your PC completely and unplug the power cord from the wall. Wait at least 30 seconds before opening the case — this lets any residual charge in the capacitors drain away. Remove the side panel by unscrewing the thumbscrews or bolts on the back of the case.
Once the panel is off, look into the lower rear corner. The power supply will be the large metal box with the fan and the thick cable bundle. You can trace the cables from the power supply to see where they connect: the 24-pin connector goes to the motherboard, the 8-pin or 4-pin connector goes to the CPU, and smaller connectors go to graphics cards and storage drives.
What not to do with the power supply
Never open the power supply case itself. Even when unplugged, the large cylindrical capacitors inside hold an electrical charge that can cause serious injury or death. The power supply is a sealed unit — there are no user-serviceable parts inside.
Do not touch the cables or connectors while the PC is powered on. Always shut down and unplug the system before working inside the case. If you need to replace the power supply, it is safer and faster to buy a new one than to attempt any repair.
Finding the power supply model online
Once you have the model number from the label, you can search for the power supply's specifications online. Type the brand and model number into a search engine to find the manufacturer's page, which will show the exact wattage, efficiency rating, and what cables it includes. This information helps if you are upgrading your PC and need to know whether the current power supply can handle new components.
If the label is worn or unreadable, you can also identify the power supply by opening the case and photographing the label clearly. Many power supply manufacturers have support pages where you can look up a unit by serial number or photo.
Frequently Asked Questions
What does the wattage number on the power supply mean?
The wattage is the maximum amount of power the unit can deliver to your PC components at one time. A 650W power supply can handle components that draw up to 650 watts combined. If your graphics card and CPU together draw 600W, you need a power supply rated for at least that much, with some headroom left over.
Can I replace the power supply myself?
Yes, if you are comfortable opening the case and unplugging cables. Shut down and unplug the PC, remove the side panel, unscrew the four bolts holding the power supply in place, disconnect the cables from the motherboard and components, and slide the old unit out. Slide the new one in, reconnect the cables in the same order, and screw it down. Take a photo of the cable connections before you disconnect them if you are unsure.
Why is my power supply fan not spinning?
Some power supplies have a feature that stops the fan when the PC is under light load to save energy and reduce noise. If the fan does not spin at all even under heavy use, or if the PC does not turn on, the power supply may have failed and needs replacement.
How do I know if my power supply is strong enough for a new graphics card?
Check the wattage requirement listed on the graphics card's box or the manufacturer's website. Add that to the wattage your current system uses (usually 100–200W for the CPU and motherboard). If the total is less than your power supply's rated wattage with 50–100W to spare, it should work. If not, you need a larger power supply.
What happens if the power supply fails while I am using the PC?
The PC will shut down when ready. You may lose unsaved work, but the sudden shutdown will not damage your other components — they are designed to handle power loss. Once the power supply is replaced and the PC is turned back on, everything should work normally.