Formula
The wall side of the supply must cover the load divided by efficiency, then the headroom:
needed = load / efficiency × (1 + headroom)
The result steps up to 450, 550, 650, 750, 850, 1,000, 1,200, or 1,600 watts. The PC power page only adds the parts.
Assumptions
- Efficiency is the share of input power that reaches the parts.
- Headroom is extra capacity above that input.
- The catalog list is a shopping step, not a brand.
Worked example
A 500 W load at 90% efficiency with 20% headroom is sold as a 750 W supply.
Input definitions
- Component load
- The watts the parts draw. The PC power page can produce this number.
- Catalog size
- The smallest listed supply at or above the needed watts.
Limitations
This is a planning figure from the numbers you type. It is not a measurement of a network, a camera, a printer, or a power supply.
Questions
Why can the catalog size be larger than the load?
Efficiency below 100% and the headroom both increase the input. The list then rounds up.
What if nothing on the list is large enough?
The page asks you to lower the load or the headroom. It does not invent a size above 1,600 watts.