How many watts does the power supply need?

The formula

Only two parts of a machine draw serious power: the CPU and the graphics card. Everything else — board, memory, drives, fans, pump — adds up to far less than either of them, so it is counted as one lump.

draw = CPU TDP + GPU TDP + 150W, then minimum PSU = draw × 1.2

The 150W covers everything that is not the CPU or the card. The × 1.2 is 20% headroom, and it is not there for neatness — it is there for the three reasons in the next section.

Why 20% headroom

1. 1TDP is not measured power draw. It is a thermal figure manufacturers use to design coolers around, and real draw can exceed it — particularly Intel CPUs running at PL2.
2. 2Brief current spikes. Modern graphics cards have transient spikes several times their average draw for fractions of a second. A supply sized exactly will trip itself at that moment.
3. 3A PSU is most efficient at around 50% load. One running flat out all the time is hotter, louder, and shorter-lived.

The card maker’s figure always wins

Graphics card makers usually publish a "recommended PSU" on their own spec page. That number comes from real measurement including the spikes the formula above cannot see, so this site always takes whichever is higher — the calculation or the manufacturer’s figure.

A clear example: Gigabyte GeForce RTX 4090 GAMING OC 24GB with AMD Ryzen 7 7800X3D calculates to 864W, but GIGABYTE specifies 1000W — so this site recommends 1000W as the manufacturer does, not 864W as the formula does.

Note the third row, rounded up from 700W to 750W: 700W is not a size shops sell. The recommendation is always rounded up to a size you can actually buy.

Enough watts is not the end of it — check the connectors

Forgotten more often than wattage: the power connectors. Many recent graphics cards use a 16-pin (12VHPWR) connector that older or budget supplies simply do not have. All the watts in the world will not help if nothing plugs in.

In this catalogue MSI GeForce RTX 4070 SUPER VENTUS 3X 12GB uses a 16-pin connector, while MSI MAG A650BN 650W 80+ Bronze has only two 8-pin leads — the wattage passes easily, but this pair cannot power the card. The checker reports that as an error, not a warning.

One more thing that has to match: the PSU’s own size

Power supplies come in several body sizes. The common ones are ATX (standard), SFX and SFX-L (for small cases). A case states which of them it accepts.

A small case such as Cooler Master MasterBox NR200P takes only SFX and SFX-L — an ATX supply will not go in, and you need something like Cooler Master V850 SFX Gold instead: the same 850W in a smaller body.