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The wattage question has a simple answer and an unhelpful amount of folklore around it. Here is the arithmetic, and the one thing it does not capture.
If you would rather have the number than the method, the compatibility checker does this arithmetic for your parts, including the headroom.
Free tool PC Build Compatibility Checker — Will your GPU, PSU, cooler and case actually fit together?The calculation
Start with the graphics card. In almost every build this is the largest single draw and the most variable. Use the figure the manufacturer publishes for the card itself.
Add the processor. Use the maximum power figure rather than the nominal one, because the nominal figure describes sustained operation and the maximum is what you are sizing for.
Add roughly seventy-five watts for everything else. Motherboard, memory, drives, fans and lighting together come to less than people assume, and this allowance is generous for a typical machine.
Then add headroom. Aim to land between half and three-quarters of the supply capacity at full load.
What the arithmetic misses
Modern graphics cards do not draw power smoothly. They draw it in extremely short bursts that can reach well above the card rated figure, over intervals so brief that no software you are running will ever show them to you.
A supply without the headroom to absorb those bursts does not gently underperform. Its protection circuitry does its job and cuts power, and you experience a machine that restarts in one particular game and nowhere else.
This is the single most common cause of a build that seems fine until it is not, and it is why the honest advice is to buy above the calculation rather than at it.
Where this leaves you
Most single-card builds with a mid-range graphics card land comfortably in the same capacity bracket, and most builds with a high-end card land one bracket above it. Our power supply buying guide covers what to check once you have that figure, our best power supplies list units for the common brackets, and our picks for the best budget power supplies cover the lower brackets specifically. If your parts list is unusual, or you want the arithmetic done against your specific components rather than against a generic example, the compatibility checker will do it and show its working.
Questions people ask
How much headroom should I leave?
Aim for the machine at full load to sit somewhere between half and three-quarters of the supply capacity. That range is where units are most efficient, where their fans are quietest, and where the components inside age slowest. Below half you are paying for capacity you will never use, which is merely wasteful. Above three-quarters you are running the unit hard every day, and a supply that spends its life hot is a supply that fails earlier than it should.
Do transient spikes really matter?
They do, and they are the reason a supply that looks adequate on paper can shut a machine down in one specific game. Modern graphics cards draw power in short bursts far above their rated figure, over intervals too brief to show up in any monitoring software you are likely to run. A supply with protection circuitry that reacts to those spikes will cut power to protect itself, which presents as a system that restarts under load. Headroom is what absorbs them.
Does a bigger supply use more electricity?
No. A power supply delivers what the components ask for, not what its label says it can provide, so an idle machine draws roughly the same from the wall regardless of the unit fitted. The only difference is efficiency at a given load, and because efficiency peaks around the middle of a unit range, a larger supply lightly loaded can be marginally less efficient than a smaller one working comfortably. The effect is small enough that it should not change your decision.