The power supply is the one part of a PC that can take every other part with it when it fails, and it is also the part people size by guesswork. Online calculators hand out a number, forum advice adds 200 watts to be safe, and graphics card boxes print a different figure again. We size power supplies for custom builds and upgrades every week at our Somerville shop, so here is how the math really works in 2026: where the watts go, why a 2025-era graphics card can trip a supply that is rated for more than it draws, what the calculators do well, and the rules we use before a PSU goes on the bench.
Two parts decide the number: the graphics card and the processor. Everything else in a typical build adds 50 to 100 W combined.
| Graphics card | Card power (maker's rating) | Maker's required system power | Connector |
|---|---|---|---|
| Nvidia RTX 5070 | 250 W | 650 W | 2 x 8-pin via adapter, or one 300 W+ PCIe Gen 5 cable |
| Nvidia RTX 5070 Ti | 300 W | 750 W | 2 x 8-pin via adapter, or one 300 W+ PCIe Gen 5 cable |
| AMD Radeon RX 9070 XT | 304 W typical board power | 750 W | 2 x 8-pin |
| Nvidia RTX 5080 | 360 W | 850 W | 3 x 8-pin via adapter, or one 450 W+ PCIe Gen 5 cable |
| Nvidia RTX 5090 | 575 W | 1000 W | 4 x 8-pin via adapter, or one 600 W PCIe Gen 5 (12V-2x6) cable |
Card power and required system power are from Nvidia's RTX 50-series product pages and AMD's RX 9070 XT specification page. The "required system power" figures assume a typical CPU and are the minimum the card maker will stand behind, not a target to hit exactly.
On the processor side, the number on the box is a starting point, not a ceiling. AMD rates the Ryzen 7 9800X3D at 120 W and the Ryzen 9 9950X at 170 W, and both can pull more under all-core loads before the motherboard's power limits step in. Intel's Core Ultra 9 285K carries a 125 W base rating with a 250 W maximum turbo power. A gaming build rarely pushes the CPU and GPU to their peaks at the same moment, which is why the card makers' system figures work for most people, and why a workstation that renders on all cores while the GPU is busy needs to be sized differently. Our workstation build guide covers that case.
The rest of the system is smaller than people think: a motherboard, two sticks of memory, an NVMe drive and half a dozen fans usually total well under 100 W. RGB lighting, a pump for an all-in-one cooler, and USB devices charging off the PC add a little more. Round it to 100 W and you will not be wrong.
This is the part the calculators cannot see, and it is the reason the ATX 3.0 and 3.1 standards exist.
Modern graphics cards do not draw a steady 300 W. They draw in bursts, with spikes that last microseconds and reach well above the rated figure. Igor's Lab measured this behavior when the ATX 3.0 standard arrived and explained what the new specification demands: a compliant supply has to survive power excursions of 200% of its rated output for 100 microseconds, 180% for a millisecond, and 160% for 10 milliseconds. An older supply that was designed for steady loads can hit its over-current protection during one of those spikes and shut the PC off in the middle of a game, even though the average draw never came close to the label.
That is the failure we see most often on the bench with a "big enough" but older PSU: a random black screen under load, no error, and a power supply that tests fine at idle. Intel's ATX Version 3 design guide, now at revision 2.1a, is the document the industry builds to. Along with the excursion tolerance it defines the 12V-2x6 connector (the revised 16-pin plug) and the PCIe 5.1 power cable that current Nvidia cards use natively.
The calculators are useful. They are just a floor, not an answer.
Seasonic's wattage calculator and be quiet!'s PSU calculator both work the same way: you pick the CPU, the graphics card, the drives and fans, and the tool sums the manufacturer-rated peak consumption of each part, then adds a margin. For a normal build that lands within a few dozen watts of what we measure at the wall, and it is a fast sanity check before you buy.
A calculator cannot tell you whether a supply meets the ATX 3.x excursion spec, whether its capacitors have aged out after seven years in a dusty case, or whether it has the right native connector for the card. It also cannot judge quality: a no-name 850 W unit and a well-reviewed 850 W unit get the same score. Those are the reasons a PSU fails in practice, and none of them show up in the number.
80 Plus Gold tells you how efficiently a supply converts wall power at a few fixed loads. It says nothing about ripple, protection circuits, fan noise, or transient handling. Cybenetics runs the stricter modern program: its ETA certification grades efficiency across a fuller load range and its LAMBDA certification grades noise, and a unit's Cybenetics report is public. When we pick a supply for a build, we read that report and the independent reviews, not the sticker. Tom's Hardware's best power supplies guide for 2026 is a good starting list because every pick there was lab-tested.
The rules we apply to every custom build and every graphics card upgrade at the shop.
| Rule | What it means in practice |
|---|---|
| Start from the card maker's number | Nvidia's and AMD's "required system power" already includes a normal CPU. For a gaming build, that figure is the minimum we will install; we usually go one step up. |
| Plan to run at 50 to 70% load | Supplies are most efficient and quietest in the middle of their range, and the headroom absorbs transients. A build that games at 450 W is happiest on a 750 W unit. |
| ATX 3.1 with a native 12V-2x6 cable for any RTX 50 card | The adapter in the box works, but a native cable from the PSU maker removes a failure point on a plug that is already carrying a lot of current. |
| Known brand, lab-tested model, long warranty | Ten-year warranties on good units are common because the makers expect them to last. A supply with no reviews and a three-year warranty is a gamble with every other part in the case. |
| Reuse an old PSU only if it passes | We test capacity and protections, check the age, and confirm the connectors. An older, pre-ATX 3.0 unit with a new high-end card is usually a no, even if the wattage looks fine. |
| Size for what the PC will become | If a graphics card upgrade is likely in two years, we buy the supply for that card now. A PSU is the one part that is annoying to swap later and cheap to oversize a little today. |
⚡ Upgrading a graphics card and not sure your supply can take it? Bring the PC in. We test the current unit, check the connectors and the age, and tell you plainly whether it stays or goes before you buy the card.
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Most gaming builds need a quality 650 W to 850 W supply. Nvidia lists required system power of 650 W for the RTX 5070, 750 W for the RTX 5070 Ti, 850 W for the RTX 5080 and 1000 W for the RTX 5090; AMD lists 750 W for the RX 9070 XT. Those figures include a typical CPU. We usually install one step above the minimum so the unit runs in its efficient 50 to 70% range.
They are a good floor. Seasonic's and be quiet!'s calculators add up manufacturer-rated draws and land close to real measurements for normal builds. What they cannot judge is transient spikes, the age and quality of a specific unit, or whether it has the right connector, which are the things that actually cause power supply failures.
ATX 3.0 and 3.1 are Intel's current power supply design specifications. They require a supply to tolerate brief power spikes of up to 200% of its rating and define the 12V-2x6 (16-pin) connector used by current Nvidia cards. If you are buying an RTX 50-series card, an ATX 3.1 supply with a native 12V-2x6 cable is the right choice.
No. A cheap 1000 W unit with weak protections and old capacitors is worse than a well-built 750 W unit. Wattage is one spec; transient handling, ripple, protection circuits and build quality are what keep the rest of your parts safe. Buy a lab-tested model from a known brand and size it so your build sits around 50 to 70% load.
It only means the unit hit certain efficiency numbers at fixed loads. It does not measure ripple, noise, protection or transient response. Cybenetics' ETA and LAMBDA certifications are stricter and publish full test reports, and independent lab reviews are the best guide to quality.
Sometimes. If it is a quality unit under about five years old, meets the card's required wattage, and has the right connectors, it can stay. An older pre-ATX 3.0 supply paired with a new high-end card is a common cause of shutdowns under load. We test the unit before any graphics card upgrade and tell you either way.
Bring the build list or the PC. We will size the supply, pick a lab-tested unit with the right cables, and install it so the rest of your parts are protected.
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