The fastest way to check your power supply

The simplest check is to look at your power supply unit itself. Open your computer case and find the metal box (usually at the bottom rear) with cables running from it. Look for a label on the side or back — it will list the wattage, model number, and manufacturer. Write down the wattage number; that is what you need to know first.

If you cannot open the case or do not want to, Windows and Mac both show you basic power information without opening anything. On Windows, right-click the battery icon in the taskbar and select "Power settings" to see what your system reports. On Mac, hold Option and click the battery icon in the menu bar. Neither method tells you the exact wattage of your power supply, but both show whether your system is drawing power normally.

For a more complete picture — especially if your computer is running slowly or shutting down unexpectedly — you need to know three things: the rated wattage of your power supply, how much power your components actually use, and whether the power supply is delivering that power reliably.

Key Takeaways

  • The wattage rating is printed on a label on the power supply unit itself, usually on the side or back of the metal box inside your case.
  • Windows shows power draw in Settings > Power & Battery, and Mac shows it in the battery menu, but neither reveals your power supply's rated capacity.
  • A power supply that is too weak for your components will cause random shutdowns, freezes, or failure to start.
  • Third-party tools like GPU-Z and CPU-Z can estimate how much power your current setup uses, which you can compare against your power supply's rating.

Finding the wattage rating on your power supply

The label on your power supply lists everything you need. Open your case (usually four screws on the back panel) and locate the power supply — it is the large metal box with thick cables. The label is typically on the side facing into the case or on the back. Look for a line that says "Max Output" or just lists a number followed by "W" (for watts).

Common wattages are 450W, 550W, 650W, 750W, and 850W. Write this number down. If the label is worn or missing, search online for your power supply model number (also on the label) and you will find the spec sheet.

If you have a laptop, the power supply is external — the brick that plugs into the wall. The wattage is printed on that brick. Laptops typically use 45W to 140W depending on the model.

Checking power draw with software tools

GPU-Z and CPU-Z are free programs that show you how much power your graphics card and processor are using right now. read GPU-Z from techpowerup.com and CPU-Z from the same site. Run them while you are using your computer normally, and they will display live power consumption in watts.

These tools do not measure your entire system — they only show the GPU and CPU — but those two components use the most power. If your CPU is drawing 65W and your GPU is drawing 120W, you are using roughly 185W plus another 50W or so for the rest of your system (motherboard, drives, fans). That total should be well below your power supply's rated wattage, with room to spare.

A healthy system uses no more than 80 percent of its power supply's capacity under full load. If your components add up to 500W and your power supply is rated 550W, you are too close to the limit. If they add up to 500W and your power supply is 750W, you have good headroom.

Signs your power supply is failing or too weak

If your computer shuts down without warning during gaming or heavy work, your power supply may be failing or undersized. Random freezes, especially during graphics-heavy tasks, point to the same problem. A computer that will not start at all, or starts and when ready shuts down, often has a dead power supply.

Unusual noises from the power supply — a high-pitched whine or loud fan noise — suggest the unit is working too hard or the fan is failing. If you smell burning plastic or see scorch marks on the label, the power supply is damaged and should not be used.

To rule out other causes, restart your computer in Safe Mode (Windows: hold Shift while clicking Restart; Mac: restart and hold Shift). If the problem does not happen in Safe Mode, the issue is likely software or a driver. If it still happens, the power supply is a stronger suspect.

Testing power delivery with a multimeter

A multimeter is a cheap tool (under $20) that measures electrical current. If you own one, you can test whether your power supply is delivering the correct voltage to your motherboard. Set the multimeter to DC voltage and touch the black probe to a black wire (ground) and the red probe to a red wire (12V rail) on any connector inside your case.

The reading should be between 11.4V and 12.6V. If it reads much lower — say 10V or below — the power supply is failing. Do this test while the computer is running and under load (run a game or stress test) for the most accurate result.

If you are not comfortable opening the case or using a multimeter, skip this step. The software checks and the label check are usually enough to diagnose the problem.

When to replace your power supply

If your power supply is more than five years old and you are experiencing shutdowns or freezes, replacement is often the cheapest fix. Power supplies degrade over time, and a unit that was adequate when new may not deliver full power after years of use.

If your components draw more power than your supply is rated for, you must replace it before the problem gets worse. A power supply running at or above its limit will fail faster and may damage your motherboard or other parts on the way down.

When you buy a replacement, choose a unit rated for at least 20 percent more than your estimated system power draw. If your system uses 400W, buy a 500W or 550W supply. This headroom keeps the unit cool and extends its life. Look for 80 Plus Bronze certification or better, which means the unit is reasonably efficient.

Frequently Asked Questions

How do I know if my power supply is dead?

A dead power supply will not turn on the computer at all, or the computer will start and when ready shut down. The fan inside the power supply will not spin. If the computer powers on but shuts down after a few seconds, the power supply may be failing but not completely dead.

Can a weak power supply damage my computer?

Yes. A power supply that is too weak will cause random shutdowns and freezes, which can corrupt files and damage the hard drive. Over time, the stress can also damage the motherboard and other components. If you know your supply is undersized, replace it before problems start.

What does 80 Plus certification mean?

80 Plus means the power supply converts at least 80 percent of wall power into usable computer power, wasting the rest as heat. Bronze, Gold, and Platinum ratings are higher tiers of efficiency. A certified unit runs cooler and lasts longer than an uncertified one.

Is it safe to open my computer case to check the power supply?

Yes, as long as the computer is powered off and unplugged. Do not touch any components inside while the power is on. If you are uncomfortable opening the case, you can look up your computer model online and find the power supply specs that way.

Can software tell me my power supply's wattage?

No. Software can only show you how much power you are using right now, not the rated capacity of your power supply. You must read the label on the unit itself or search for the model number online to find the wattage rating.