The formula
recommended = sum of draws × (1 + headroom)
Where it comes from
Adding each part’s draw gives the peak, but a power supply should not run at its limit. Its efficiency curve peaks around 50 % load, and that is also where it is quietest and lasts longest, because the capacitors take less punishment. Hence the 25 to 30 % headroom, which is not a sales trick. The graphics card is almost always the heaviest component, and its instantaneous draw can go well above nominal during millisecond spikes: that is what shuts down a system with a tight PSU even when the arithmetic checks out.
How to work it out by hand
- Add the load draw of every component
- Add the headroom over that peak
- Step up to the next standard wattage
- Check what percentage of load it would run at
What is worth knowing
80 Plus certification measures efficiency, not quality: a good Bronze beats a bad Gold. What is worth looking at is the current delivered on the 12 volt rail, which is what the processor and graphics card draw from, and whether the manufacturer publishes the actual platform — many brands rebadge other people’s designs. Efficiency matters less than people think on a home machine: the difference between Bronze and Gold on a desktop is a few euros a year. What does matter a great deal is over-current and over-voltage protection, because a cheap unit that fails can take the rest of the system with it.