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Basic PSU buyers guide:

I have seen enough conversation, (and whats worse, misunderstanding) about PSU's around here, that I thought that I would try writing a brief guide for those of you that are either upgrading, or are building a completely new system.

PSU Basics:

Everyone knows that a PSU is rated in terms of watts. The question is always “will this ____ PSU be enough for my system?”. The thing is, not all companies rate their PSU's the same, and in that way, not all Watts are created equal.

In reality, as long as the PSU has enough amperage on the 12v rail, it should easily have enough wattage for whatever it is that you might need. The reason that I say this, is that the two components that pull the most power (CPU and GPU), both run off of the 12v “rail” of the PSU (the part that puts out 12v, rather than the 5v or 3v rails).

Most companies will list how much amperage the PSU has on the 12v rail on the sticker on the side of the PSU. If not, then you just have to do some simple math. Amps = watts/volts, so its easy enough to figure that out. For instance, my Antec Earthwatts 430w PSU has 360w on the 12v rail, so you can see how 360w/12v = 30amps.



Now, for where things are really misunderstood. I have seen a lot of posts that talk about a PSU having so many 12v rails with 18amps, and they simply add them together for the total amperage of the unit. This is not the way that works. You must do it the same way that we did before, which is to find the total amount of power available to the 12v rails as a whole (almost all multi-rail systems are actually just a split single rail), and do the same math.

For instance, my 430 is listed as having two 17amp 12v rails, which would give me 34 amps if you just added them together. The trick is that while those rails function semi-independently, the total that is drawn from them cannot be greater than the 360w that was stated before, which is why the real amperage is 30amps, not 34.

In general, this is how it breaks down as far as wattage and amperage for different types of builds. Though obviously this is more to give you a basic idea of what you should be looking at, than being a definitively guide to which PSU to buy.

Midrange Single card rigs 300-400w is fine, 24-32amps

Performance single card rigs 400-600w, 32-40 amps

Performance multi-card rigs 600-1Kw, 40-60amps

HERE is the best PSU calculator that I know of. Notice that it will tell you the wattage that you need, but will also tell you about this whole "amperage thing" that I keep talking about. Between this forum and that calculator, I think you should get a pretty good idea of what you need.

A good quality PSU is also important. The PSU plays more of a part than most people might think. The PSU is what supplies the power for everything (I know, obvious, but hear me out). If the PSU is of poor quality, it can provide very “dirty” power to the components (power that is bouncing around a lot in the acceptable range). This can hamper your OC (because the CPU isn't getting a clean, stable supply of power, its jumping all over the place), as well as potentially damage components. In fact, a bad PSU is one of the easiest ways to loose hardware. If it for some reason spikes, it could easily zap everything in your computer. And one time I helped an OCN member out that had sporadically unstable OC's... turns out it was the fact that his computer was plugged into the same circuit as his refrigerator, and apparently that caused the power to fluctuate enough that it made the OC unstable. The problem stopped when he put the computer on another circuit.

I think this is also a good place to mention the fact that having at LEAST a good surge protector. Even if you have a good PSU, a power surge can take it, and usually your motherboard out in just a few seconds (Its most common to have a surge take out those two components, more specifically, the MOSFETS or other caps on the motherboard). Having a good surge protector, or even better, a UPS will help make sure that your computer lasts as long as it should, as well as making it more stable for even better OC's.

Some good manufacturers of PSU (aka, ones that have a good reputation) are: Corsair, PC Power & Cooling, Antec, Seasonic, Delta, OCZ and Thermaltake. Of course there are others, but here in the states, those are some of the better known ones. As a general rule of thumb, look at the efficiency of the PSU that you're looking at. If the PSU is very efficient (better than 75%) then its probably at least decent (it takes good components to make an efficient PSU, the ones with low efficiency are often not that great of quality).

Other than that, just look for the PSU that has the right connections for what you need (note: Modular PSU's are convenient, but there has been some question of the reliability of the connections , especially after repeatedly taking them apart, so just be aware of that). Sleeving is also nice, and not only helps out with the aesthetics of the case, but makes cable management easier, although its not really essential.

Final notes:
A dying PSU will usually just cause the computer to turn off at random, usually without warning. This can also be caused by the PSU overheating. A PSU that is underpowered will usually make itself known when the PSU is under the most load (stressing both the CPU and the GPU at the same time.... like intense gaming or benchmarks). And a PSU can be bad, even if the fans and lights of the computer turn on. That is another common thing that I see. People will say that the PSU can't be bad, because the fans spin... that just isn't true. A fan will operate with huge differences in voltage... a CPU will not.

Hopefully this helps clear things up a bit for some of you. I know that most of you probably knew all of this, but I think enough will find this useful as it is, and it will be part of a more comprehensive OC guide that I am working on in conjunction with thlnk3r and Joe. Because we all know that OCin isn't just about the CPU, the rest of the system also comes into play.

Images will be coming soon.

Cheers!

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NickOulton

Nice guide, Repped

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Devon

nice guide, thats way i use apevia power supplies, they are for the extreme gamer since my extreme edition pentium 4 made the apevia go pop.

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JadedFloridian

Awesome guide, simply reading this would solve ~70% of all the questions asked here

Where's the sticky?

Afromee

Sticky ! Nice guide man , helped me clear up some uncertainties about PSU !

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Biatch

It is good.

parityboy

Yep it's good for sure, so +rep coming your way. Also highlights that PSU specs can be misleading.

Assume you pulled 17A from one of your 12V rails, that's 17 * 12 = 204W. 360W - 204W = 156W. 156W / 12V = 13A. 13A is what's left over on the other rail. So, basically if one rail is fully loaded, the other one is left "lopsided", so to speak. A more "honest" PSU would have enough juice to allow each rail to be fully loaded.

It'd be nice if systems like ESA could show this sort of thing on a gauge - the relative loads on each of the rails.