Power Supply Basics: What a PSU Does in Your PC

A power supply unit (PSU) is the part of your computer that takes electricity from your wall outlet and changes it into the power your PC parts need. Your wall outlet sends alternating current (AC) at 120 volts in the United States (or 230 volts in other countries). Your computer parts need direct current (DC) at much lower voltages. The PSU handles this conversion.

Inside your PC, different parts need different voltages. Your graphics card and processor need 12 volts. USB ports and some storage drives use 5 volts. Memory sticks and motherboard chips run on 3.3 volts. The PSU creates all these voltages from the single power source coming from your wall.

Why Your PSU Matters

Many people spend lots of time picking out processors and graphics cards, then grab whatever power supply fits their budget. This thinking is backwards. If your PSU doesn't deliver clean, steady power, your expensive parts won't work right.

A low quality power supply might deliver 11.4 volts when your graphics card needs 12 volts. That small difference is enough to cause crashes, visual glitches, or system freezes. The symptoms look like a bad graphics card or faulty memory, so people often replace the wrong parts.

How Power Supplies Work

Understanding what happens inside your PSU helps explain why quality matters. Here's the basic process:

PSU Wattage

Wattage tells you how much total power your PSU can deliver. A gaming PC with a powerful graphics card might need 750 watts or more, while a basic office PC could run fine on 400 watts.

To pick the right wattage, add up how much power all your parts use and then add about 20% extra. This extra room, called headroom, helps your PSU run cooler and last longer. It also gives you space to upgrade parts later without buying a new power supply.

Typical Power Use by Part

Efficiency Ratings

The 80 Plus certification system tells you how well your PSU converts power. Higher ratings mean less wasted energy and lower electricity bills. When a PSU wastes energy, that energy becomes heat, which makes your computer run hotter.

Here's what efficiency means in real terms: if you have a 90% efficient PSU delivering 600 watts to your parts, it pulls about 667 watts from the wall. The extra 67 watts becomes heat. A 80% efficient unit would pull 750 watts from the wall for the same 600 watts of output, wasting 150 watts as heat.

When Efficiency Matters Most

PSUs work best at around 50-60% of their rated capacity. Running at very low loads (under 20%) or very high loads (over 90%) reduces efficiency. For the best results, pick a PSU that runs at 50-70% of its capacity during your typical use.

Modularity Types

Modular PSUs let you connect only the cables you need. This makes cable management easier and helps air flow better inside your case. PSUs come in three types based on their cables:

• Non-Modular: All cables are attached and cannot be removed. Cheapest option, but unused cables take up space.
• Semi-Modular: Essential cables (main motherboard and CPU power) are fixed. Other cables can be removed if not needed.
• Fully Modular: All cables can be removed. Most expensive, but offers the cleanest build.

Protection Features

Quality PSUs include several safety features to protect your system from electrical problems:

Look for PSUs that list all of these protections. Budget units often skip some of these features, which puts your parts at risk.

Common PSU Problems

Here are the most common power supply issues and how to handle them:

System Won't Turn On

• Check that the power cable is plugged in at both ends
• Make sure the PSU switch on the back is set to "on" (the "I" symbol)
• Verify the wall outlet works by testing with another device
• Check that all power connectors inside the PC are fully seated

Random Shutdowns

• Check if the PSU fan is spinning (blocked fans cause overheating)
• Verify your PSU has enough wattage for your parts
• Look for dust buildup that might cause overheating
• Check system temperatures using monitoring software

Strange Noises

• A clicking or grinding sound usually means the fan is failing
• High-pitched whining (coil whine) is annoying but usually harmless
• Buzzing or humming could signal capacitor problems
• If noise comes with burning smell, shut down immediately

Testing Your PSU

Before replacing a PSU, you can test it to confirm it's actually the problem:

The Paperclip Test

This basic test checks if the PSU turns on at all:

1. Unplug the PSU from all parts inside your PC
2. Find the 24-pin motherboard connector (the largest one)
3. Use a paperclip to connect the green wire to any black wire
4. Plug in the PSU and flip the switch on
5. If the fan spins, the PSU has basic power

Multimeter Test

For more accurate testing:

1. Set your multimeter to DC voltage
2. Test each voltage rail (3.3V, 5V, 12V)
3. Values should be within 5% of the rated voltage
4. The 12V rail should read between 11.4V and 12.6V

Choosing the Right PSU

When shopping for a power supply, focus on these key factors:

• Calculate your needs: Add up your parts' power use and add 20% headroom
• Check efficiency: 80 Plus Bronze is the minimum for most builds, Gold for gaming PCs
• Look at modularity: Semi-modular offers good value, fully modular gives cleanest builds
• Verify protections: Make sure OVP, UVP, OCP, and SCP are all included
• Check warranty: Five years or more suggests the maker trusts their product
• Read reviews: Look for professional reviews that test voltage stability under load