Graphics card and PSU requirement

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?

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.
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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.

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.