How Much Power Supply Do You Need for Your PC?
How to size a PC power supply: add the processor and graphics card, leave headroom, respect the card maker's minimum, and skip watts you will not use.
In short: Add the processor's and graphics card's power, add 75 W for everything else, multiply by 1.25 for headroom and round up to the next 50 W, and never go below the card maker's own recommendation. Our builder does this sum for you.
The power supply is the part people most often get wrong, in both directions. Too small, and the PC crashes under load or will not start. Too large, and you pay for capacity you never use. Here is the method our builder uses, so you can check it yourself.
The method
- Find the peak draw. Add the processor's power rating, the graphics card's power rating, and 75 W for the motherboard, drives and fans.
- Add headroom. Multiply by 1.25, so the power supply is not running flat out at the busiest moment.
- Round up to the next 50 W.
- Check the card maker's minimum. Graphics card makers publish a recommended power supply size. Never go below it, even if the sum comes out lower.
The recommended size is the larger of the two numbers.
Two examples
An everyday gaming PC. A processor rated at 65 W and a mid-range card rated at 165 W:
- Peak: 65 + 165 + 75 = 305 W
- With headroom: 305 x 1.25 = about 381 W, rounded up to 400 W
- If the card maker recommends 550 W, the answer is 550 W.
A high-end PC. A processor rated at 125 W and a card rated at 285 W:
- Peak: 125 + 285 + 75 = 485 W
- With headroom: 485 x 1.25 = about 606 W, rounded up to 650 W
- If the card maker recommends 750 W, the answer is 750 W.
In both cases the card maker's figure wins. That is the pattern: with a graphics card, its own recommendation is usually the number that matters.
Why headroom matters
Power supplies are most efficient and run coolest and quietest in the middle of their range. A supply running near its limit is hotter, noisier and more likely to trip protection when a sudden load spike arrives. Headroom is cheap insurance.
Why more is not always better
A 1200 W supply in a PC that peaks at 400 W works, but it costs more, and it runs in the part of its range where it is slightly less efficient. Buy what you need plus headroom, and spend the difference on a better graphics card or more storage.
Quality counts as much as wattage
Two supplies with the same wattage are not equal. Look at:
- The efficiency rating. 80+ Gold or Platinum supplies waste less power as heat than basic ones.
- The connectors. The graphics card must have the right power connectors. High-end cards often use a newer high-power connector, which good supplies include.
- The size. Most cases take a standard ATX supply, and small cases need the smaller SFX kind. The compatibility checklist covers this.
What the builder does
Choose a processor and a graphics card, and the builder works out the recommended wattage and blocks any power supply below it. If you change a part and the supply is no longer enough, it says so and suggests the closest one that is. The same check runs again at checkout.
Ready to build? Open the builder or browse the ready-made builds, and pay with Bitcoin or any coin on the coins page.