From OCAU Wiki
The Big PSU FAQ
Here is a quick question and answer section, to answer lots of typical queries and misconceptions.
Q: Hang on, isn't a PSU's wattage all that matters?
A: This was partially true for older computers, but recently theres been a big change, suddenly the big power users (videocard,cpu) are all powering off the 12V rail, so it becomes the limiting factor rather than a wattage rating. Wheras before the CPU would power off the 5V rail, with older PSU's designed with that in mind. So try to run that older PSU on the newer system and it often has issues since it wasn't designed for the newer system with an entirely different rail draw (almost all off 12V rail, instead of a mix). Besides most of today's systems have trivial wattage usage, see SilentPCreview for measured wattage draws of 6 different computers. It's quite obvious from the P4 dual core, that even a quality 350w unit could power it provided it had a big enough 12V rail, since the 12V rail draw was 17A, but only a 223W total wattage draw (DC). so therefore wattage ratings are generally now a poor guide to how good a PSU is (order of importance roughly is: PSU Brand, 12V rail capacity, needed connectors (24pin in particular), then wattage). As a 'rule of thumb', unless the combined 12V rail capacity (times the amps by 12 if its listed in amps) is at least the wattage rating minus about 40w (eg 30A x 12V = 360w for a 400W psu, or 38 x 12 = 456w for a 500w psu - in both cases about 40w), the 12V rail will be the limiting factor for modern systems (not wattage rating). For PSU's that don't meet this grade its best to judge their rating off the 12V combined rating + 40w. So for a Hyper 580w (12V combined = 30A or 360w), its effectively a 400w psu for modern systems.
Q: But hang on, I had a Codegen 300W and it wouldn't run my 6800gt system, yet I grabbed the codegen 550w watt unit and it works now! Surely thats good enough proof you need big power supply's for today's systems?
A: The problem is not the wattage, but PSU brand and 12V rail. You'll notice codegen is listed as a 'not recommended PSU'. Also the 300w model has a tiny 12V rail and poor voltage regulation, unsuitable for even a light modern systems, while the 550w has a bigger 12V rail (but still not very big). But even the $65 Antec smartpower 2.0 350w has a bigger 12V rail than the codegon 550w, another reason to avoid those generic brands (on top of reliabilty).
Q: What about PSU weight? I hear thats a good 'rule of thumb' for PSU quality
A: Nope. Most of the weight in a psu is the transformer, and the chassis. But most of the parts critical to psu quality (voltage regulators, capacitors etc), all weigh very little (eg. a cheap crappy capacitor weighs the same as a good one etc). Extreme Overclocking Explains further.
Q: What are minimum load requirements that Seasonic / Antec Neo HE and other high efficiency PSU's have?
A: The issue here is for the psu to operate properly, it needs a 12V load of around 2A or more. A very light system, such as onboard graphics / one hard drive (typical htpc), may not meet at startup this if the motherboard delays powering the cpu for a second, so psu instantly shuts down. So for such systems, avoid the really high end high efficiency psu's (such as seasonic s12 500w +, neo HE 500w +), the lower end (eg seasonic s12 430w) shouldn't have this problem. SilentPCReview explains further. Note that the latest revisions of the Antec neo He and Seasonic S12 series fix this issue by adding internal load inside the psu to allow psu to startup with no external load, and then automatically disconnects once external load is avaliable.
Q: I read a (Tomshardware/Anandtech?) review and it said a 7800gtx draws 250W, so hows my 400W PSU supposed to power it once you add the CPU and hard drives to that?
A: First of all, that graph was the power consumption of the WHOLE PC (cpu/graphics card included), and includes the PSU heat losses since it was measured on the 240VAC/120VAC side. multiply that by roughly 3/4 and thats a ballpark DC wattage draw.
Q: My PSU isn't SLI certified/isn't labelled as 'prescott ready' etc. Can I use it?
A: Lets put things in perspective here. First of all a PSU has no knowledge of whether its an intel, or a nvidia, or ATI based PC, it only knows about the loads it has to deliver. The real guide is whether the PSU has sufficient 12V rail ratings, appropriate connectors (eg 24pin, pci-e power), is a good brand, and to a lesser extent has a reasonable wattage rating. 'prescott' or 'p4' ready are just marketing labels, and whether an PSU is SLI certified mainly depends on whether they send the check to Nvidia or not.
Q: I own an Antec smartpower 2.0 power supply and its rather loud, even when the PC is idling. what can I do?
A: unplug the fan moniter cable from the motherboard. Theres a bug that when it's plugged in, the fan always runs at full speed (regardless of load), disconnecting it will return it to the desired temperature controlled fan speed.
Q: I have a 20pin PSU and a 24pin motherboard. Should I use a 20-24pin adapter, or replace the PSU?
A: First of all, don't use a 20-24pin adapter, they don't help at all (the missing pins are connected electrically on the motherboard). Just plug it in so the pins matches up to the motheroard, there'll be 4 pins empty on one side. Whether it will work stable or not depends on the videocard, overclocking, motherboard. Also older PSU's tend to have a small 12V rail, so may not be suitable for that reason. If unsure, replace the PSU, especially if you planning on overclocking. Theres also the issue of group regulation, however if the 12V rail is decent on your older 20pin psu (at least 18A, more if your running high end videocard / intel P4), then it should go ok, assuming a decent psu brand. If your motherboard has extra connectors (eg 4 pin molex), plug these in as well to help power pcie videocard.
Q: Hang on, isn't dual rails better as it gives me a cleaner supply of 'juice' to the cpu?
A: Generally no. Dual rails doesn't mean there completely seperate rails (its still only 1 12V power source)- you can measure the resistance between the 12V1 (molex) and 12V2 (atx12v) and it will still be a short. The only difference is 20A current limiters to comply with EU safety regulations, which are don't do a thing unless the 20A is reached. Critically, theres still only one regulator/set of filters for the 12V2 rails, so the cpu won't get any cleaner current, since its really only a SINGLE 12V rail from a user point of view (unless 20A is reached). Besides, the cpu isn't powered straight off the 12V rail, it's the motherboard that takes the dirty 12V and regulates it down to whatever the cpu needs (eg 1.4V), so a dirty input voltage isn't to be worried about (since the motherboard will clean it up).
Q: Do you need super tight rails (eg 1% regulation) to get maximum overclock?
A: Generally no, theres little to no benefit in having super tight rails as far as cpu overclock is concerned. The motherboard used has a far bigger influence. Furthermore, looking at the OCAU 13 PSU roundup, you'll notice that for prime95 stability, theres only a 2mhz fsb between the budget Antec Smartblue 350w and super tight rails Antec TrueControl 550w. The differences are small enough to be within the bounds of error of the test (suggesting no benefit at all). However there is one benefit that is quite useful. Tight rails generally means NO group regulation (independant voltage regulation), which is a good thing, since it ensures minimal to no cross loading requirements.
Q: What exactly is group regulation?
A: Group regulation is a way to make a PSU cheaper by tying up the 3.3V/5V/12V regulation together. The effect is that it leads to cross loading requirements, and can sometimes result in situations where a PSU can handle the load, but can't keep the voltages in spec due to a badly designed group regulation circuit (or a group regulation circuit designed for another type of system, such as a server). On the other hand, a NON group regulated PSU (independently regulated), has basically no cross loading requirements, so you can load the PSU up in just about any way (within individual rail and total wattage limitations) and voltages will be within spec. Xbitlabs reviews show the difference vividly. Compare the excellent ocz powerstream 470w with the medicore OCZ modstream 520w. The ocz modstream has a very nasty cross loading graph, and to draw 25A off the 12V rail (typical of an SLI system), requires a 5V/3.3V load of at least 100W! (modern systems will not get near 100W for 3.3V/5V load). On the other hand, the ocz powerstream 470w is basically perfect, we can draw almost no 5V/3.3V current at all and still draw as much as 33A off the 12V rail (so it will run any SLI system you throw at it). For a more technical explenation, read up on Xbitlabs guide.
Q: So what are cross loading requirements?
A: Basically its a minimum required load on each of the main rails (usually 3.3V,5V), to keep the other rails in spec. An extreme example is for a silverstone zeus 650w (older revisions mainly), it has the a cross loading requirement of: 10A minimum load on 5V rail if 12V rail load is between 30A-38A. So in this case, to load the 12V rail up to 30A, you need a 5V load of at least 10A as well. If you don't, the voltages may not be in spec (out by more than 5%). For a visual representation, again compare the Compare the Xbitlabs graphs of the excellent ocz powerstream 470w with the medicore OCZ modstream 520w.
Q: The videocard I have said needed a 26A 12V rail for my videocard and my PSU (eg Antec Smartpower 2.0 500W) only has a 17A 12V rail. What do I do?
A: The confusion of multiple 12V rails strikes again!. You'll see on the PSU label theres a 12V1 and 12V2, so add the ratings together, which makes 36A! Nothing to worry about!
Q: I own a HTPC, but can't seem to find a small form factor PSU with a 22A 12V rail. what do I do?
A: As already mentioned above the 12V rail guides are very general, so don't always apply to specific cases. For a htpc, the cpu and videocard draw is very small and theres no overclocking, so even a 15A 12V rail is plenty assuming a quality brand such as Shuttle. Same applies for low end machines like an athlon64 3000+/6200 @ stock, theres no need for a big PSU, but if your using an ATX case you might as well for future proofing, and since $55 gets you a 25A 12V rail (silverstone st-365 or Coolermaster xp 430w), as cheap as you'll get for brand name PSU's.
Q: What are these ATX 1.0, 1.1, 2.0, 2.2, BTX etc. motherboard standards that my PSU supports?
A: Check out the IBM Standard and Specs article: The ATX case and power supply