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?
Solution:
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.
