How to Choose a PSU Wattage for Your Build

Add up what the parts can actually draw, leave a fifth of the supply spare, then check the card maker has not asked for more.

By Efe Arda Ergez · Published 21 September 2026

The short answer

Take the processor’s real power limit, not its TDP.
Add the graphics card’s board power.
Add an allowance for everything else — about 44 W for a typical build.
Divide by 0.8 so the supply runs at no more than 80% of its rating, and round up to the next size sold.
If the card maker recommends a bigger supply, use theirs.

For a Ryzen 7 9800X3D with an RTX 5070, that is 162 W + 250 W + 44 W = 456 W. 456 W ÷ 0.8 is 570 W, so the rule gives a 600 W unit. NVIDIA recommends 650 W for the card, which is larger, so 650 W is the size to buy.

Why the processor’s TDP is the wrong number

TDP (thermal design power) is a cooling specification: roughly the heat a cooler should be able to move under a sustained load. It is not the most power the chip will draw. Modern processors boost well beyond it for as long as temperatures and the motherboard allow — AMD’s limit is called PPT (package power tracking), Intel’s the Maximum Turbo Power.

The gap can be large. A Core i9-14900K is sold as a 125 W part and draws up to 253 W. Size a supply on the headline figure and you are 128 W short before the graphics card is counted: with an RTX 5070, a TDP sum points to a 550 W unit when the real load calls for 700 W.

Headline TDP against the power limit each processor actually reaches, largest gap first
Processor	TDP	Power limit	Undercount
Core i7-13700K	125 W	253 W	+128 W
Core i7-14700K	125 W	253 W	+128 W
Core i9-13900K	125 W	253 W	+128 W
Core i9-14900K	125 W	253 W	+128 W
Core Ultra 7 265K	125 W	250 W	+125 W
Core Ultra 9 285K	125 W	250 W	+125 W
Ryzen 9 7900X	170 W	230 W	+60 W
Ryzen 9 7950X	170 W	230 W	+60 W
Ryzen 9 9950X	170 W	230 W	+60 W
Ryzen 9 9950X3D	170 W	230 W	+60 W
Core i5-13600K	125 W	181 W	+56 W
Core i5-14600K	125 W	181 W	+56 W
Core i5-12400F	65 W	117 W	+52 W
Ryzen 7 7800X3D	120 W	162 W	+42 W
Ryzen 7 9800X3D	120 W	162 W	+42 W
Ryzen 9 7950X3D	120 W	162 W	+42 W
Ryzen 9 9900X	120 W	162 W	+42 W
Ryzen 5 7600X	105 W	142 W	+37 W
Ryzen 7 5800X3D	105 W	142 W	+37 W
Ryzen 7 7700X	105 W	142 W	+37 W
Ryzen 9 5950X	105 W	142 W	+37 W
Core Ultra 5 245K	125 W	159 W	+34 W
Ryzen 5 5600X	65 W	88 W	+23 W
Ryzen 5 7600	65 W	88 W	+23 W
Ryzen 5 9600X	65 W	88 W	+23 W
Ryzen 7 9700X	65 W	88 W	+23 W

What “the rest of the system” means

Everything besides the processor and card is small individually but adds up. The allowance used here and in the calculators is itemised:

Motherboard: 25 W
Memory module × 2: 6 W
NVMe SSD: 7 W
Case fan × 3: 6 W

A second SSD, a SATA drive or an all-in-one cooler’s pump each add a few watts more; the calculator lets you count them. Case lighting and USB devices are small enough not to change the size you buy.

Why leave a fifth of the supply spare

A supply running near its full rating runs hotter, louder and less efficiently, and has nothing left for the short spikes modern graphics cards produce — bursts of a few milliseconds well above their rated board power. Sizing so the continuous load is at most 80% of the rating leaves room for those spikes, for parts ageing, and for the upgrade you may make in two years.

Efficiency is a separate question. An 80 Plus rating describes how much power is lost as heat on the way from the wall to the parts, not how much the supply can deliver. The 456 W build above pulls about 507 W from the socket through an 80+ Gold unit and 536 W through an 80+ Bronze one. That matters for your electricity bill and a UPS, not for which wattage to buy.

When to follow the card maker instead

NVIDIA and AMD publish one recommended supply per card. They have to assume some processor, so for a modest build their figure is often larger than the sum above — take it anyway: they know how their board handles transient spikes, which a sum of rated powers does not model.

The reverse also happens. With a Core i9-14900K, an RTX 5090 build reaches 872 W; the 80% rule asks for 1200 W, more than NVIDIA’s 1000 W. The maker’s number is a floor for their card, not a ceiling for your build, so the rule is to take whichever of the two is larger.

Popular builds: full-load draw, the size the 80% rule gives, the card maker's recommendation and the larger of the two
Build	Full load	80% rule	Card maker	Buy
Ryzen 5 9600X + RTX 5060	277 W	450 W	550 W	550 W
Ryzen 7 9800X3D + RTX 5070	456 W	600 W	650 W	650 W
Ryzen 7 9800X3D + RX 9070 XT	510 W	650 W	750 W	750 W
Core i5-14600K + RTX 5070 Ti	525 W	700 W	750 W	750 W
Ryzen 7 9800X3D + RTX 5080	566 W	750 W	850 W	850 W
Core i9-14900K + RTX 5070	547 W	700 W	650 W	700 W
Ryzen 7 9800X3D + RTX 5090	781 W	1000 W	1000 W	1000 W
Core i9-14900K + RTX 5090	872 W	1200 W	1000 W	1200 W

What this method does not cover

It is a sizing estimate from rated figures, not a measurement. Overclocking, factory-overclocked cards with raised power limits and unusually power-hungry motherboards all push the real load up; if any of those apply, or if the result lands within a few watts of a size boundary, go one size up. Quality matters as much as wattage: a well-reviewed unit from a reputable maker at the right size is a better buy than a larger, cheaper one.

Calculators used in this guide

Processors: AMD and Intel published processor specifications. Cards: Manufacturer specifications, shared with the GPU power dataset. Rest of the system: component class allowances, itemised on every page. Every figure on this page is computed at build time from the same data and code as those calculators; the formulas are on the Methodology page.