Every PSU calculator gives you a number. Almost none of them tell you where the number came from, which is a problem, because the difference between a 650 W answer and an 850 W answer is a hundred dollars and a rebuild.
This PSU calculator shows its working. Put in the two figures that matter and it prints every line of the sum, the source behind each figure it supplies, and the things it cannot tell you. No part of it was measured on a bench here: this site reads published specifications and what owners report, and says which is which.
In this guide
The PSU calculator
NVIDIA calls it Total Graphics Power, AMD calls it Typical Board Power. It is on the card's own specification page.
The figure the card's maker publishes as the minimum system power requirement. Put 0 if you cannot find it.
Intel calls it Maximum Turbo Power, AMD calls it PPT (Package Power Tracking). Not the base TDP, which is the lower of the two numbers.
SSDs and hard disks together.
Count an all-in-one cooler's pump as one.
Lower means a bigger supply running cooler and quieter. 60 is the figure explained below.
How that is worked out, line by line
- Everything added up471 W
- Sized so the peak sits at your chosen share785 W
- The larger of that and the card maker's own minimum785 W
- Your peak would then be this much of it55 %
On those numbers you are in the 700 to 850 W band, which is the section below.
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Where every number here comes from
No part of this was measured on a bench here. This site works from published specifications and from what owners report, and every number below is either one you typed in or a published figure with its source named next to it.
The figures this page supplies
- Motherboard, memory, chipset and USB: 60 W. An allowance, not a measurement: a desktop board with memory and its own controllers is commonly quoted in the tens of watts, and 60 is set high enough to cover a well-populated one.
- Each drive: 8 W. An allowance covering the thirstiest of them: a 3.5-inch hard disk spins up at single-digit watts and an NVMe SSD peaks around 8 W under sustained writes.
- Each fan or pump: 5 W. An allowance: a 120 mm case fan is rated in the low single digits and an all-in-one pump a little above them.
Why each step is the step it is
- Everything added up:
gpu_w + cpu_w + board_w + drives * drive_w + fans * fan_w. The peak figures the makers publish for the two parts that dominate the draw, plus a stated allowance for everything else. No part of it is measured here. - Sized so the peak sits at your chosen share:
parts_w / (load_pct / 100). 80 PLUS certifies efficiency at 20%, 50% and 100% of rated load and a supply's efficiency peaks near the middle of that range, so a build whose peak lands near 60% of the supply runs in its efficient band. Intel's ATX 3.1 design guide additionally requires a supply to ride out power excursions well above a graphics card's rated draw, which is headroom you cannot see in a component total. - The larger of that and the card maker's own minimum:
Math.max(sized_w, gpu_min_psu). The card's maker has tested its own card on its own reference system and publishes a minimum. Where that is higher than the arithmetic, the maker's number wins. - Buy at least this supply:
atLeast(answer_w, [450, 500, 550, 600, 650, 700, 750, 850, 1000, 1200, 1300, 1600]). Supplies are sold in a ladder of sizes, so the answer is rounded up to the next one actually on sale rather than left as a number nobody can buy. - Your peak would then be this much of it:
Math.round(parts_w / recommend_w * 100). The component total divided by the supply you would buy. Useful as a sanity check: far below 40% means you are paying for capacity you will not use.
What this cannot tell you
Every calculator has an edge, and a page that hides its edge is worse than no page at all. Here is this one's.
- It adds up published figures. Nothing here is measured from a wall socket, and this site has never had any of these parts in its hands.
- Board partners change a card's power limit, and an overclock or an undervolt moves it again. Read the figure on the box you are actually buying.
- It says nothing about the quality of a supply: connectors, cable lengths, rail layout, the fan curve and how long the warranty runs are not watts and are not in this arithmetic.
- A transient spike lasts microseconds and no calculator can see whether a particular supply rides it out. That is what the ATX 3.1 rating on the box is for.
The supplies I would look at
These follow from the answer above and nothing else. Each heading below covers one span of results, so read the one the calculator pointed you at and ignore the rest.
650 W or less
A build in this range is a mid-range card and a mainstream processor. I have not covered a supply this size on this site yet, so I am not going to name one: pick an ATX 3.1 unit from a maker with a long warranty and read a review that actually measured it.
700 to 850 W
This is where most current builds land, and it is the one size this site has written about.
Above 850 W
A top-end card and a high-core-count processor together. Nothing on this site covers a supply this size yet, so read a review with a load tester in it rather than a calculator.
If the number surprised you, the line-by-line working above is the place to look: it is usually the card's own minimum doing the work, not the sum of the parts. That is the part a PSU calculator without its working cannot show you.
Questions about the PSU calculator
How many watts does my PC need?
At its peak, about the sum of what the makers publish for the graphics card and the processor, plus a stated allowance for everything else. The build this PSU calculator opens with has a 250 W card and a 125 W processor; add 60 W for the board and memory, 8 W for each of its 2 drives and 5 W for each of its 4 fans, and the peak comes to 471 W. That is what the parts draw together, not the size of supply to buy, which is sized so the peak uses only part of it. Put in your own two figures and the working redoes the sum.
What size power supply do I need?
The first size on sale that is at least your peak draw divided by the share of the supply you want the peak to use, or the card maker's own minimum when that is larger. On the example build, 471 W divided by 60% is 785 W. The card maker's published minimum is 650 W, so the larger of the two is 785 W, and the next size sold is 850 W, at which the peak uses 55% of the supply. Supplies come in steps, which is why the answer is a size you can buy rather than the raw figure.
Why not buy a power supply that just covers the total?
Because a supply is most efficient in the middle of its range, and it has to ride out spikes a total does not show. 80 PLUS certifies efficiency at 20%, 50% and 100% of rated load and efficiency peaks near the middle, so this calculator sizes the peak at 60% of the supply by default. Intel's ATX 3.1 design guide also requires a supply to ride out power excursions well above a graphics card's rated draw. Sized only to cover the 471 W total, with no card maker's minimum in the way, the example build would get a 500 W supply at 94% of its rating at peak; at a 60% share it gets 850 W at 55%.
Does the graphics card maker's minimum PSU matter?
Yes, and when it is higher than the arithmetic, it wins. A card's maker publishes a minimum system power figure for its own card, and this calculator takes the larger of that and its own figure. On the example build the arithmetic's 785 W is already above the maker's 650 W. For a smaller build, a 115 W card with a 65 W processor, the arithmetic comes to 460 W, which on its own would round up to a 500 W supply; a maker's minimum of 550 W takes the answer to 550 W. Put 0 in that box if the card's page gives no figure.
Can a PSU calculator tell me whether a power supply is any good?
No. It works out a size, and size is all it knows. Connectors, cable lengths, rail layout, the fan curve and how long the warranty runs are not watts, and they are not in this arithmetic. A transient spike lasts microseconds, and no calculator can see whether a particular supply rides it out: the ATX 3.1 rating on the box is the maker's published statement that it should, and a review that put the unit on a load tester is where its quality shows.
Sources
- 80 PLUS certification: Sets out the efficiency a supply must reach at 20%, 50% and 100% of its rated load, which is why sizing for a peak near the middle of that range is worth doing.
- Intel ATX Version 3.1 desktop power supply design guide: Defines the power excursions a modern supply has to ride out when a graphics card spikes above its rated draw, which is the headroom a plain component total misses.
- NVIDIA GeForce graphics card specifications: Publishes each card's Total Graphics Power and its minimum system power requirement.
- AMD Radeon graphics card specifications: Publishes each card's Typical Board Power and recommended supply.
- Intel processor specifications (ARK): Publishes Processor Base Power and Maximum Turbo Power for every part.
- AMD processor specifications: Publishes TDP, and PPT where the socket defines it.
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