5

Note: This is intended to become a canonical post.

I got a dedicated graphics card of type XXX and it claims I need at least a YYY Watt PSU.

How big does my PSU really need to be?

5
+50

There's a few things to look at. The average "recommended power requirement" is a safe bet with a typical system though as always, your mileage may vary. I might choose to be nutty enough and somehow jam in a top of the line video card on a 5W processor and an emmc, or have a system with 6 hard disk drives or dual processors.

The recommended power requirement is a safe bet with most systems though - they take into account the typical processor and video card, and if your video card is power hungry, you don't want to cheap out on that.

Another thing to look at is that a higher wattage card may need more than one PCIe power input, and a higher wattage power supply might have that.

tldr: Don't cheap out on your power supply

As for the actual ratings, I tend to use a PSU calculator and add a bit of headroom for what wattage to buy.

What could happen with an underpowered PSU? While most PSUs are most efficient in a given power range, your PSU is going to be straining to meet the needs set on it. You may have short term issues, like your system unable to get enough power to start up, or possibly shutting down under load, to longer term issues like overheating of your PSU and failure.

I'd actually consider the other option - getting a more powerful PSU with more than the 'minimum' amount of power I need. Modern PSUs are more efficient at specific bands - with a higher efficiency at ~50% to 80%, with better PSUs more efficient at 50%. It makes a ton of sense to get an 80+ PSU close to, and with a slightly higher rating than needed. Most PSUs are 80% efficient over most of their range and you arn't going to draw more power than you will use. A overspecced PSU would run slightly cooler (win!) possibly quieter (if the fans are thermally throttled) and possibly use less power - assuming you get a bronze or better rated PSU.

Other things to look at would be whether its single or multiple rail. This isn't something I've looked at in detail so I'll defer to one of the authorities online on power supplies, the entertaining, and educational Johnny Guru and see what he has to say. And he says its a non issue

The bottom line is, for 99% of the folks out there single vs. multiple +12V rails is a NON ISSUE. It's something that has been hyped up by marketing folks on BOTH SIDES of the fence.
....

...

it should be a non-issue assuming that the PSU has all of the connectors your machine requires and there are no need for "splitters" (see Example 1 in the previous bullet point).

He does bring up an additional point - that If using SLI or Crossfire, is the unit SLI or Crossfire certified (doesn't matter if a PSU is certified for one or the other as long as it has the correct connectors. If it passed certification for EITHER that means it's been real world tested with dual graphics cards in a worst case scenario).

Which may be a good idea if you're going that way.

6

The PSU must be capable of more than YYY watts, but that's far from being the whole story.

First, the fact that a PSU has YYY watts written on it, does not mean that it is capable of really delivering YYY watts continuously. Most PSUs will fall short of this maximum, sometimes even by as much as 10%, depending on their quality.

Second, the card is not alone in the computer, and each and every component of the computer draws power. One should also include in the list USB devices that don't have their own power-cables.

To help with the task of estimating the power needs of the computer, one can resort to on-line calculators. I have used such calculators before to get a rough idea of the power needs when ordering a new computer.

If the manufacturer of your computer supplies such a calculator, it is advised to use that one. Otherwise, here are a few such that I found :

Newegg.com - Recommended Power Supply Calculator
eXtreme Power Supply Calculator - can take overclocking in account
MSI Power Supply Calculator

There are more calculators to be found on the Internet. As results vary between calculators, I would suggest to use more than one, take the higher result, then add at least 10%-20% as safety margin.

[EDIT] A comment below has convinced three readers to downvote this answer. I hereby quote from Wikipedia :

A power supply that is self-certified by its manufacturer will claim output ratings that may be double or more than what is actually provided. To further complicate this possibility, when there are two rails that share power through down-regulating, it also happens that either the 12 V rail or the 5 V rail overloads at well below the total rating of the power supply. Many power supplies create their 3.3 V output by down-regulating their 5 V rail, or create 5 V output by down-regulating their 12 V rails. The two rails involved are labeled on the power supply with a combined amperage limit. For example, the 5 V and 3.3 V rails are rated with a combined total amperage limit. For a description of the potential problem, a 3.3 V rail may have a 10 A rating by itself (33 W), and the 5 V rail may have a 20 A rating (100 W) by itself, but the two together may only be able to output 110 W. In this case, loading the 3.3 V rail to maximum (33 W), would leave the 5 V rail only be able to output 77 W.

a 900-watt power supply with the 80 Plus Silver efficiency rating (which means that such a power supply is designed to be at least 85-percent efficient for loads above 180 W) may only be 73% efficient when the load is lower than 100 W. [...] For a comparison, a 500-watt power supply carrying the 80 Plus Bronze efficiency rating (which means that such a power supply is designed to be at least 82-percent efficient for loads above 100 W) may provide an 84-percent efficiency for a 100 W load, wasting only 19 W.

The above also means that choosing a PSU that has much more capacity than your needs will waste electricity on its own functioning, and especially under low consumption (idling).

Knowing what any given PSU is capable of in reality is complicated, and empirical measurements on each rail are the only real test. Therefore it is advised to choose a PSU leaving a comfortable margin above the total calculated consumption.

Wikipedia further advises :

Power supplies are designed around 40% greater than the calculated system power consumption.

And this is even more than the conservative 10%-20% figure I gave above. The balancing act is not simple between having a PSU that is sufficient for all needs without risking a burnout, while at the same time not of too high capacity so as not to waste too much electricity while the computer is idle. But if in doubt, too much is safer than too little.

  • Wrong wrong wrong wrong wrong. You do NOT need a PSU capable of more than YYY and MOST PSUs will deliver 10-20% MORE than the rated power. The ONLY PSUs that do not deliver the rated output are faulty PSUs. – qasdfdsaq Feb 11 '16 at 21:26
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    @qasdfdsaq: I gave above some quotes from Wikipedia, which support my answer and more. – harrymc Feb 12 '16 at 8:47
  • what does that mean? "Power supplies are designed around 40% greater than the calculated system power consumption." My system and my PSU are completely independent.. Also, don't cheap out on PSU, buy best in given class/wattage. – aaaaaa Feb 12 '16 at 8:55
  • @aaaaaa: It means taking the calculated power consumption and ensuring the PSU has 40% more. Less is possible If one knows the exact properties of a given PSU. A good quality PSU will be more reliable, in the sense that its actual output will be nearer the labelled capacity. It will still not reach 100%, unless it is secretly designed to allow more than is labelled, but it will then consume more power for its own functioning. The PSU is labelled for total capacity on both rails, but the graphical card is only connected to one. – harrymc Feb 12 '16 at 9:09
  • @aaaaaa: Power consumption and the ability to deliver power are indeed independant. PSU's tend to perform less efficient in some cases though. E.g. a 100W drawing system on a 150 Watt capable PSU might draw 125 Watt from the wall socket, loose 25Watt while converting that to other voltages and deliver 100Watt to the system. One the same 100 Watt using system a 1200Watt PSU will also deliver 100Watt, but it probably draws more power from the wall socket. More power lost this way means higher electricity bills, more heat (louder fans), and no advantage. – Hennes Feb 13 '16 at 11:11
3

The "??? Watts needed" figure that comes with a graphics card is a very rough guesstimation and is frankly quite misleading. It makes assumptions about how many Watts the rest of the system would consume, as well as some extra on top of that. This works in most most cases where a user has a typical setup but is way off on cases where people have a non-standard setup.

On the other hand, most power supplies cannot actually provide the wattage they claim to provide.

The real question should be "how beefy should my PSU be". To answer that you need to first look up the actual power requirements of the video card on a reliable review site. Then you need to choose a quality PSU manufacturer and buy a PSU from them that can provide that much wattage on their PCIe power cables.

A good way to guesstimate it is to look at the power cables. The graphics card will have 1 or 2 power sockets on it, which take in 6 or 8 pins each. While in theory a graphics card could take in power from both the PCIe slot and the external plugs, all modern graphics cards take their power entirely from the external power cables rather than the PCIe slot, and are generally capped below 300 Watt.

If you do not care for doing quite a lot of research on it, just buy a 600 Watt PSU made by a well known brand . It will be good for any single graphics card system.

1

Don't cheat on the minimum power requirements if you are planning to boot an operating system from a live USB flash drive on the same computer. The minimum power requirements provide a margin of safety because you need a margin of safety to provide enough power for full usage of the other features of your computer. A computer is not worth as much if you can't install/reinstall the operating system because the USB flash drive isn't getting an uninterrupted supply of power.

When you buy a laptop, all of its components were selected by engineers to work properly together. When you are building your own system, you have to think like an engineer which is unpleasant unless you are an engineer. Checking the power supply requirements with an online power supply calculator will simplify this task.

Power requirements for graphics cards

The following are the minimum power requirements using a PSU with a single +12V rail. Also these requirements are for a system containing basic components (unless otherwise noted). The more add-in cards and hard drives that a system has, the more amperage draw on the 12V rail.

nVidia
GeForce GTX Titan X - 38A and a 600W PSU minimum
GeForce GTX 980ti - 38A and a 600W PSU minimum
GeForce GTX 980 - 30A and a 500W PSU minimum
GeForce GTX 970 - 28A and a 500W PSU minimum
GeForce GTX 960 - 20A and a 400W PSU minimum
GeForce GTX TITAN Z - 42A and a 700W PSU minimum
GeForce GTX TITAN Black - 42A and a 600W PSU minimum
GeForce GTX TITAN - 38A and a 600W PSU minimum
GeForce GTX 780ti - 42A and a 600W PSU minimum
GeForce GTX 780 - 42A and a 600W PSU minimum
GeForce GTX 770 - 42A and a 600W PSU minimum
GeForce GTX 760 - 30A and a 500W PSU minimum
GeForce GTX 750ti - 20A and a 400W PSU minimum
GeForce GTX 750 - 20A and a 400W PSU minimum
GeForce GTX 740 - 20A and a 400W PSU minimum
GeForce GTX 730 - 20A and a 300W PSU minimum
GeForce GTX 690 - 46A and a 650W PSU minimum
GeForce GTX 680 - 38A and a 550W PSU minimum
GeForce GTX 670 - 30A and a 500W PSU minimum
Geforce GTX 660ti - 24A and a 450W PSU minimum
Geforce GTX 660 - 24A and a 450W PSU minimum
Geforce GTX 650ti Boost - 24A and a 450W PSU minimum
Geforce GTX 650ti - 20A and a 400W PSU minimum
Geforce GTX 650 - 20A and a 400W PSU minimum
GeForce GT 640 - 20A and a 350W PSU minimum
GeForce GT 630 - 20A and a 350W PSU minimum
GeForce GT 620 - 18A and a 350W PSU minimum
GeForce GT 610 - 16A and a 300W PSU minimum
GeForce GTX 590 in SLi - 78A and a 1000W PSU minimum
GeForce GTX 590 - 46A and a 700W PSU minimum
GeForce GTX 580 - 40A and a 600W PSU minimum
GeForce GTX 570 - 35A and a 550W PSU minimum
GeForce GTX 560 Ti in SLi - 42A and a 700W PSU minimum
GeForce GTX 560 Ti - 31A and a 500W PSU minimum
GeForce GTX 560 - 24A and a 4500W PSU minimum
GeForce GTX 550 Ti - 24A and a 400W PSU minimum
GeForce GT 520 - 18A and a 300W PSU minimum
GeForce GTX 470 and 480 in SLi - 55-60A and a 850W PSU minimum
GeForce GTX 480 - 42A and a 600W PSU minimum
GeForce GTX 470 - 38A and a 550W PSU minimum
GeForce GTX 465 in SLi - 46A and a 700W PSU minimum
GeForce GTX 465 - 30A and a 500W PSU minimum
GeForce GTX 460 in SLi - 42A and a 650W PSU minimum
GeForce GTX 460 - 26A and a 450W PSU minimum
GeForce GTS 450 in SLi - 35A and a 600W PSU minimum
GeForce GTS 450 - 24A and a 450W PSU minimum
GeForce GT 430 - 22A and a 350W PSU minimum
GeForce GTX 295 in Quad SLi - 70A and a 1000W PSU minimum
GeForce GTX 295 - 50A and a 700W PSU minimum
GeForce GTX 285 - 46A and a 550W PSU minimum
GeForce GTX 280 in SLi - 55A and a 800W PSU minimum
GeForce GTX 280 - 40A and a 550W PSU minimum
GeForce GTX 275 in SLi - 50A and a 800W PSU minimum
GeForce GTX 275 - 40A and a 550-600W PSU minimum
GeForce GTX 260 OC in SLi - 50A and a 700W PSU minimum
GeForce GTX 260 OC - 40A and a 550W PSU minimum
GeForce GTX 260 in SLi - 50A and a 700W PSU minimum
GeForce GTX 260 - 38A and a 500W PSU minimum
GeForce GTS 250 - 24A and a 450W PSU minimum
GeForce GT 240 - 18A and a 300W PSU minimum
GeForce GT 220 - 18A and a 300W PSU minimum
GeForce 9800 GX2 - 35A and a 550W PSU minimum
GeForce 9800 GTX+ - 32A and a 500W PSU minimum
GeForce 9800 GTX - 25-30A and a 450W PSU minimum
GeForce 9600 GT - 26A and a 450W PSU minimum
GeForce 9600 GSO - 26A and a 450W PSU minimum
GeForce 8800 Ultra - 35A and a 500W PSU minimum
GeForce 8800 GTX – 30A and a 450W PSU minimum
GeForce 8800 GTS 512MB - 28A and a 500W PSU minimum
GeForce 8800 GTS 320/640MB – 26A and a 400W PSU minimum
GeForce 8800 GT in SLi - 36A and a 500W PSU minimum
GeForce 8800 GT - 26A and a 450W PSU minimum
GeForce 8600 GT/GTS - 24A and a 350W PSU minimum
GeForce 8500 GT - 22A and a 350W PSU minimum
GeForce 8400 GS - 18A and a 350W PSU minimum
GeForce 7950 GX2 – 27A and a 400W PSU minimum
GeForce 7950 GT – 22A and a 350W PSU minimum
GeForce 7900 GTX – 22-26A and a 350-400W PSU minimum
GeForce 7900 GTO – 22-26A and a 350-400W PSU minimum
GeForce 7900 GT – 22-26A and a 350-400W PSU minimum
GeForce 7900 GS – 22-23A and a 350W PSU minimum
GeForce 7800 All - 26A and a 400W PSU minimum
GeForce 7800 GS AGP - 20A and a 400W PSU minimum
GeForce 7600 GT – 22-26A and a 350W PSU minimum
GeForce 7600 GS – 15-17A and a 350W PSU minimum
GeForce 7300 GT – 15A and a 300W PSU minimum
GeForce 7300 GS – 15A and a 300W PSU minimum

ATI/AMD
Radeon R9 Fury X - 32A and a 600W psu minimum
Radeon R9-390X - 30A and a 550-600W psu minimum
Radeon R9-390 - 30A and a 550-600W psu minimum
Radeon R9-380 - 28A and a 500W psu minimum
Radeon R9-370 - 17A and a 450W psu minimum
Radeon R9-295X2 - 52A and a 800W psu minimum
Radeon R9-290X - 33A and a 600W psu minimum
Radeon R9-290 - 31A and a 550W psu minimum
Radeon R9-285 - 25A and a 500W psu minimum
Radeon R9-280X - 30A and a 550W psu minimum
Radeon R9-280 - 25A and a 500W psu minimum
Radeon R9-270X - 24A and a 500W psu minimum
Radeon R9-260X - 19A and a 450W psu minimum
Radeon HD 7990 - 48A and a 750W psu minimum
Radeon HD 7970 Ghz Edition - 31A and a 550W psu minimum
Radeon HD 7970 - 30A and a 500W psu minimum
Radeon HD 7950 - 25A and a 500W psu minimum
Radeon HD 7870 - 23A and a 500W psu minimum
Radeon HD 7850 - 21A and a 500W psu minimum
Radeon HD 7790 - 21A and a 500W psu minimum
Radeon HD 7770 - 19A and a 500W psu minimum
Radeon HD 7750 - 16A and a 400W psu minimum
Radeon HD 6990 - 45A and a 750W PSU minimum
Radeon HD 6970 - 34A and a 550W PSU minimum
Radeon HD 6950 - 30A and a 500W PSU minimum
Radeon HD 6870 X2 - 40A and a 600W PSU minimum
Radeon HD 6870 CrossfireX - 38A and a 650W PSU minimum
Radeon HD 6870 - 28A and a 500W PSU minimum
Radeon HD 6850 CrossfireX - 35A and a 600W PSU minimum
Radeon HD 6850 - 25A and a 450W PSU minimum
Radeon HD 6770 CrossfireX - 33A and a 600W PSU minimum
Radeon HD 6770 - 25A and a 450W PSU minimum
Radeon HD 6790 - 25A and a 450W PSU minimum
Radeon HD 6670 - 17A and a 400W PSU minimum
Radeon HD 5970 CrossfireX - 70A and a 900W PSU minimum
Radeon HD 5970 - 50A and a 600W PSU minimum
Radeon HD 5870 CrossfireX - 55-60A and a 700W PSU minimum
Radeon HD 5870 - 40A and a 500W PSU minimum
Radeon HD 5850 CrossfireX - 55-60A and a 650W PSU minimum
Radeon HD 5850 - 35-40A and a 500W PSU minimum
Radeon HD 5830 - 35-40A and a 500W PSU minimum
Radeon HD 5770 - 34A and a 500W PSU minimum
Radeon HD 5750 - 30A and a 450W PSU minimum
Radeon HD 5670 - 26A and a 400W PSU minimum
Radeon HD 5570 - 24A and a 350W PSU minimum
Radeon HD 5450 - 22A and a 300W PSU minimum
Radeon 4890 - 32A and a 550W PSU minimum
Radeon 4870 X2 CrossfireX - 67A and a 1000W PSU minimum
Radeon 4870 X2 - 43A and a 600W PSU minimum
Radeon 4870 - 32A and a 500W PSU minimum
Radeon 4850 - 30A and a 450W PSU minimum
Radeon 4830 - 26A and a 400W PSU minimum
Radeon 4770 Crossfire - 34A and a 750W PSU minimum
Radeon 4770 - 26A and a 400-500W PSU minimum
Radeon 4670 - 26A and a 400W PSU minimum
Radeon 4650 - 24A and a 350-400W PSU minimum
Radeon 4550 - 20A and a 300W PSU minimum
Radeon X3870 X2 - 30A and a 550W PSU minimum
Radeon X38xx series - 28A and a 450W PSU minimum
Radeon X2900 XT Crossfire - 45A and a 800W PSU is recommended for high-end rigs
Radeon X2900 XT - 35A and a 500W PSU is recommended for high-end rigs
Radeon X2900 Pro - 35A and a 500W PSU minimum
Radeon X2600 XT - 25A and a 400W PSU minimum
Radeon X2400 XT - 20A and a 350-400W PSU minimum
Radeon X1950 XTX – 20A and a 420W PSU minimum
Radeon X1950 Pro – 20A and a 420W PSU minimum
Radeon X1900 XTX – 25A and a 520W PSU minimum
Radeon X1900 XT – 22-25A and a 520W PSU minimum
Radeon X1900 GT – 22A and a 400W PSU minimum
Radeon X1650 Pro – 18A and a 350W PSU minimum

  • 3
    Heh, that is along list. But I am looking for a post where people will also explain why. E.g. a office PC (using max 100W) with a 300Watt PSU which is capable of delivering 200 Watt on a +12v rail (16 amp) will do just fine in many cases where manufacturers list minimal 500 Watt. Explaining that nice and legal (without copying from other sites) is the main goal. Explains rails in a PSU, max power from a PCI-e slot (75W), 6 and 8 pins PCI-e connectors etc is a bonus. – Hennes Feb 5 '16 at 11:45
  • My GTS 250 (450w) ran just fine on 300w. – Mazura Feb 12 '16 at 9:19
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    @Mazura Your observation is correct. The power supply recommendations are conservatively overrated to allow a for a variety of special situations that may occur, since incrementally larger power supplies are relatively inexpensive. – karel Feb 12 '16 at 9:28
1

Many graphics card ship with a requirement of a PSU of at least XXX Watt.
This is a simplification based on assuming a common setup and with a margin of error.

Let’s look into what the real requirements are:

A graphics card draws power via the PCI-e bus and optionally from additional PCI-e power (6pins/8 pins) connectors. It is allowed to draw up to 75 Watt via the PCI-e slot. This is sufficient for low end cards. Most of the time these cards do not list any extra requirements

However high end graphics cards need a lot more power than 75 Watt. In order to satisfy these power requirement we use extra power connectors, which look like this:

PCI-e express power connectors - male and female, 8,6 and 6+2 pins

The 6 pins connectors is used to supply up to 6¼ ampere at +12v (which equals 75 watt). The 8 pins connector supplies the same +12v but up to 12¼ ampera (150 Watt). If this is not enough then multiple connectors can be used.

An advantage of using power cables directly connected to the PSU is that all this power is not routed via the motherboard.

The voltage used (+12v) is choosen because higher voltage means lower amperage for the same amount of power and +12v is the highest commonly available power in regular desktop computers.

This means that we have two demands:

  1. The PSU needs to power the main system (motherboard, CPU, memory, disks, fans, ...) This is mainly +3.3v and +5 volt
  2. But it also needs to be able to supply a lot of powered on a +12v rail.

These two sets of voltages are often intertwined. E.g. a typical 400 Watt PSU might be able to supply up to 150 Watt on the normal +3.3 and +5v domains and up to 300 Watt on the 12v rail. That is a total of 450 Watt, which means both parts cannot be maxed out at the same time.

This means we need to look at the following before buying a PSU:

How much power is needed for my main system? On-line power calculators are often the easiest, but in general you need: Up to 50 Watt for the motherboard A few Watt (2-3W) per memory module 3-5 Watt per fan 5-15 Watt per HDD (typical 15 Watt at power up. (If you have lots of disks this can add up) 1-6 Watt per SSD

Add all of these up, then look up the power requirements of your graphics card. These are explicitly mentioned on most manufacturers’ sites.

Start by adding both these numbers. This will be the minimum size PSU you want to buy. Add some spare overhead. E.g. if your sum was 400Watt then round off to a 450W PSU.

You now have a starting value. Next decide on which brand and which type of PSU you want. Some no name PSU’s deliver less than their rated maximum power. If you get one of the lesser known brands add more spare headroom. You do not need to get a higher rated PSU than that unless you want to add more hardware later.

Getting a PSU which is rated for significantly more power will probably consume more power for the same amount of power delivered. This is because PSU are not equally efficient at all loads. Most of them perform best around 80% of maximum load.

Based on this, select a few candidate PSU's and check their specifications. A manufacturers' website should show all this information. A physical box typically has a setup like this:

 +-------------------------------------------------------------+
 |              Brand X model YYY - ZZZ Watt                   |
 +-------------------------------------------------------------+
 | +3.3v   +5v   +12v   |   +12v             |   +12V          |
 | 150 W   50W   10W    |  150 W             |   150 W         |
 |    Max 150 W         |                    |                 |
 +-------------------------------------------------------------+

Note the split in three sections. You can use one cable connected to the right most +12v rail And use it to power up to 150 Watt. E.g. as a 8 pin PCI-e connector, or via a splitter to two 6 pins (75W each) PCI-e connectors.

You cannot use it to split to 3 6 pins connectors. Not even if you do not connect anything to the middle +12v rail.

Added: A picture of my PSU which shows this IRL:

Picture of a PSU label

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