Summary: Power supplies are like the red-headed step-child of computer components. Nobody pays attention to them until they throw a hissy-fit or burn the house down while you're out at the movies. It doesn't have to be that way, you just have to know how to handle them. Treat them fairly and they'll be on their best behavior for as long as you both are under the same roof.
Are you building a new computer and need help picking out a power supply? Maybe your old one died and you're looking for a more appropriate replacement? This helpful guide reads like a crash course in all the basics and then walks you through the process of shopping for a PSU step-by-step. In no time at all, you will be fully prepared to make an educated purchasing decision, so read on!
Building a new computer is an exciting thing, at least for most of the people who would be reading this. Whether it’s a gaming rig, file server, home theater PC, or something simple for Mom to use for email, web browsing, and FarmVille, there’s a certain thrill associated with buying a bunch of parts from Newegg and spending an afternoon putting it all together. It’s the satisfaction of knowing you picked out every individual piece in that machine, assembled it with your own two hands, and saved some money in the process (prebuilts are for suckers!). You’ve got your case, motherboard, CPU, memory, video card, hard drive, DVD burner, and, uh… what am I forgetting? Oh, that’s right, the power supply! The thing you plug the rest of the stuff into so that it turns on, that’s no big deal, right?
Popular PSU Requirement Calculators
Now that you’ve got that figure in your head, let’s talk about what it means.
When thinking about power supply wattage, you must remember that some manufacturers advertise a higher number of watts than the unit is capable of reliably supplying. In that case, what you see on the box will actually be its “peak output,” a figure that the power supply is technically capable of, but only for very short periods of time. If your system regularly demanded that amount of power, perhaps while playing a game, it would likely damage the PSU and eventually lead to failure.
The easiest way around this is to make sure you purchase a power supply with more than enough advertised wattage, so that you have a buffer of sorts to prevent stressing it too much. This is probably what most people do, rather than commit the time and effort to learn how to pick out the proper unit. Since you’re reading this, you’re already better than them! You know what they say...
What you should be looking for in a PSU’s specifications is its continuous power output, which can be provided safely for the lifetime of the part. Good quality power supplies will clearly display this on the label, or at least mention it in the manual somewhere. You might also take note of the conditions in which that output rating was determined. Some power supplies may be rated at room temperature (20-30 degrees Celsius), but once it’s installed in your computer, the air inside the case will undoubtedly be much warmer, perhaps around 40-50 degrees C.
Heat has a direct effect on how much power the unit is capable of providing, so if the temperature of the air in your case exceeds the power supply’s operating temperature range, you can expect its wattage to decrease. The technical term for this phenomenon is “derating,” and some manufacturers will mention it in the PSU specifications. Here’s an example of what a derating curve might look like if a power supply’s operating temperature range is between -20 and 50° C, with an automatic shut-off threshold at 80° C:
W = V x A
As you can see, it combines 30 amps on the +3.3V rail, 36A on the +5V rail, and 25A on the +12V rail to come up with its total power. 99 watts plus 180 watts plus 300 watts is 579 watts, which is how they come up with that 580W total. That seems like enough power for a lot of mid- to low-end gaming rigs, doesn’t it? (If you’re wondering, the +5Vsb refers to the stand-by rail; it’s always on while the PSU is plugged in and allows the computer to be turned on by pressing a button connected to the motherboard. The -5V and -12V rails are basically obsolete.)
But notice that the label doesn’t specify whether this is peak or continuous power, nor does it tell us what operating temperatures that rating applies to. The omission of that information likely indicates that the manufacturer is being deceptive, so we should assume that that is peak power at room temperature, i.e. the best case scenario that is not at all indicative of real-world usage. Not that you should ever get anywhere near a $20 power supply anyway… Reviews of this particular unit show it struggles to output more than half of that 580W!
Here’s another label, from the Antec TruePower Trio 430W I used in one of my old computers:
It’s a bit more complicated with the multiple +12V rails (we’ll talk about those in a minute), but you can still multiply the volts and amps to figure out the total wattage. 75 plus 72.6 plus 192 plus 192 plus 192 equals… *gasp* 723.6 watts! If that’s the total wattage, why isn’t that listed here and on the box? Because that is the “theoretical” output, and since Antec doesn’t lie to you, they list the actual, continuous output of 430W, a full 300 watts less.
Part of this large discrepancy is due to the footnote you see underneath where it says “Total output power;” even though there are three +12V rails with limits of 16 amps each, the maximum load for all +12V current is 32A, only 2/3 of what we calculated before. When we account for this, we come up with about 530 watts, which sounds about right for a “peak output” figure compared to the total output they advertise. Maybe you could draw up to 500W out of this unit, but that wouldn’t be very good for its health.
This ThermalTake Toughpower PSU has four +12V rails: two with a load limit of 20 amps and two with limits of 36 amps. Altogether, it seems like that should account for 1344 watts on the +12V rails alone, but the maximum load across all four is 100A, or 1200 watts. That is also the continuous power rating for the entire PSU, so you wouldn’t actually be able to tax the +12V rails to their fullest; the +5V and +3.3V need to be accounted for first. With a power supply this huge, you most likely wouldn’t have to worry about load limitations, but it does present an interesting example for the splitting of +12V rails. That is, it’s actually two “true” +12V rails split into four “effective” ones.
The reason manufacturers split +12V rails boils down to safety. The more power you put through one rail, the more chance there is something bad will happen if it fails (like a fire). But of course, it’s cheaper to have as few rails as possible in order to save on circuitry, which is why most power supplies that claim to have multiple rails really don’t. Instead, they have multiple outputs stemming from one +12V source. For example, the Antec 430W PSU above most certainly consists of a single +12V source with a load limit of 32A, hence why the total of its three outputs are limited to that number, even though they are individually capable of delivering up to 16A.
Getting back to the ThermalTake 1200W, it is apparently composed of two 50A +12V sources split into four +12V outputs. One 20A rail is used for the motherboard and peripherals, the other 20A is for the CPU, and both 36A ones are used for the graphics cards. To prevent overloading, it’s suggested that each graphics card in a multi-GPU setup get its own +12V rail. They even help you keep track of this by labeling the plugs and cables. Aside from that, multiple +12V rails in a PSU can cause problems with wasted amperage. When one or more components connected to a rail don’t utilize all of the amps that it is capable of providing, the leftover amps are “wasted” because nothing else can access them.
This is actually where the idea of the total load across all +12V rails being less than their individual amperages added together starts to make sense. In all likelihood, you won’t be maxing out the amperage on any one rail, so with the other ones drawing from the same “pool” of amps, you’re not wasting as much. Although, that doesn’t do anything to eliminate the threat of overloading: if a component attempts to draw more amps than a single rail can supply, the PSU will shut down.
For that reason, some of the more expensive power supplies opt for a single +12V rail with a very large amount of amperage. That certainly takes some of the guesswork out of managing your power draw loads, but that is really the only advantage a single rail has over multiple ones. Aside from ensuring a power supply provides enough watts and amps to suit your needs, it’s more important to weigh the other features of the unit.
80 PLUS Certified
Most new power supplies will have one of the above logos on its packaging, which means that it has passed rigorous testing by a third party and is considered to be particularly energy efficient. At least the ones from more reputable manufacturers do, as the program is voluntary and they’re willing to pay extra to have their products tested. There’s a good rule of thumb for you: stay away from any PSU that is not 80 PLUS certified (with few exceptions). The basic white sticker means that it is at least 80% efficient, while higher tiers Bronze, Silver, Gold, and Platinum signify 82%, 85%, 87%, and 90%, respectively. Very few power supplies attain Platinum status and boy, are they expensive!
Yikes! It’d be much better if you could detach the cables you’re not using and get rid of some of that clutter, don’t you think? That’s why a lot of manufacturers use a modular cable system, so you only plug in what you need, aside from the required motherboard and CPU cables. Even better, they wrap some or all of them in a mesh sleeve to prevent individual wires from sticking out and catching on edges, as well as make them less likely to get tangled up. Here’s a good example, a uniquely-styled 650W PSU from XFX:
WTF is PFC?
Now that you’ve read through this article and have gained a general knowledge of how power supplies work, you’re ready to pick one out! To help you along in this endeavor, I’ve put together this handy walkthrough so that you don’t forget anything.
You can’t determine your power budget if you don’t know exactly what processor, video card(s), etc. will be going into this computer.
Refer to the first page for a power supply calculator. Remember that you get a more accurate estimation by inputting as much detail as possible.
The CPU and graphics cards are our primary concern here, but you will need some extra headroom for the rest of the parts. Generally Newegg or the manufacturer’s website will give you the power requirements in watts, so you must divide by 12 to get the number of amps they need. For example, a Core i7 2600K is rated for 95W, or just under 8 amps. A GeForce GTX 560 Ti uses up to 170W, or a little more than 14 amps (double that if you want two in SLI). Don’t forget what you learned about multiple rails!
You should be looking at power supplies with 50-100W more than your estimate in continuous output around 40-50° C and enough amperage on the +12V rail(s) to accommodate all of your components and then some. Check out this awesome list from XtremeSystems.org that ranks the power supply brands -- I would recommend looking at units from Tier 3 or higher that are 80 PLUS certified (and SLI/CrossFire-ready, if you’re going for a multi-GPU setup).
It’s most important that it has enough PCI-E connectors of the right type (6-pin and/or 8-pin) for your graphics card(s) because using 4-pin Molex adapters can unbalance the load on your +12V rails. Other than that, you don’t have to think about this much unless you have a LOT of hard drives or other miscellaneous stuff.
Of all the goodies PSUs can come loaded with these days, I’d say modular cabling is the most worthwhile. A large ball-bearing fan is best if you want to reduce noise, and extended warranties are always nice to have, as well. You might also see units with multiple fans to help with case cooling, LED lights for aesthetics, external voltage adjustments, premium capacitors, etc… It gets a little crazy the higher in price you go, as everyone’s trying to toss in a little something extra to get you to buy from them, so just use your best judgment.
This should be a no-brainer, but always read reviews for products that you're thinking about spending your money on. The reviews on Newegg are a decent barometer most of the time, but for power supplies, you can check out JonnyGuru.com if you want in-depth analysis and knowledgable opinions. You don't have to understand all of the tests they do, but it's easy to tell whether a PSU is good or bad depending on how they talk about it.
That’s pretty much all there is to it. Hopefully, all will go well and you won’t have to think about power supplies for a long time to come. Remember that if anything goes wrong -- maybe it’s defective, incompatible somehow, or you underestimated your energy needs -- you usually have a month to send the unit back to where you bought it for refund or replacement.
I hope you found this guide helpful in your quest to Choose the Right Power Supply. Share it with your friends and be on the lookout for more how-to articles from FiringSquad in the future. If you have any questions, comments, or suggestions on topics for upcoming guides, speak up in the comments section!
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