Power Supply Unit

Latest revision as of 14:19, 29 January 2012

The PSU, or Power Supply Unit, is responsible for converting the (AC) mains power received from the wall socket into various DC voltages used by the components of a PC.

Computer systems need a good clean energy source. When purchasing a $3000 dollar top of the range system most people neglect to also put those extra bucks into a decent reliable energy source for their system. No longer is the $20 'yumcha' PSU that comes with a $60 case sufficient, as they are noisy and unreliable (often dying after a year or two's use, taking out components with them), but more importantly are unsuited to powering modern computers due to a weak 12V rail, and don't have the right connectors (24pin atx, pcie, sata etc).

[edit] Introduction

Disclaimer: This information is just a collection of user comments. Any decisions you make from the following is your choice alone.

Basically a good rule of thumb to follow when purchasing your next PSU is to look at how much the unit costs, and the PSU brand. If you're buying a case with a '480W' PSU included for under $60 then you're almost guaranteed a problematic, and possibly dead, system. Money wise, look at spending at least $70 for a cheap reliable brand name PSU (see below) for some basic overclocking, with strong 12V rail(s). If you are getting into some more serious OC and running SLI you would probably want to invest some more money (probably in the range of $150+) and time into researching what you need.

For more information on why GENERIC power supply brands are not good, see the SPCR PSU guide and X-bit labs PSU guide, as well as jonnyGURU for an example of a PSU not handling its 'rated' loads (500w powmax psu blows up before its even loaded up to 300w)

[edit] What To Look For

Name brands (see below) are usually the best indicator of quality/reliability. Also look at the individual rail specifications to check they are suited for your computer. For newer (any motherboard with ATX12V/P4 4pin connector) computers, the 12V rail specifications (how much capacity they have, measured in 'amps') are the most important. The capacity of the 5V and 3.3V rails are very important for old P3/Athlon XP systems (ones without ATX12V/P4 4pin connector), and just about irrelevant for newer systems (This is because P3/Athlon XP motherboards power the CPU off the 5V rail, while newer motherboards power the CPU off the 12V rail, so in a newer system, there are no big power users on the 5V/3.3V rail at all, only the 12V rail). Use the PSU calculator below to work how big a PSU (wattage and 12V amps) you need.

PSU Wattage and Rails Calculator

Multiple 12V rails explained:
With ATX12V 2.0 and later, 2 or more 12V rails was introduced to comply with safety regulations (no more than 20A / 240W on any one user accessible wire). The way this is done in every PSU except for a couple of very high end PSU's such as Enermax Galaxy 1kw is that there is still only one 12V power source, which is then split in two or more 'rails' via current limiters (typically set to around 18-20A, even if PSU label says its less). This means that when comparing multi 12V rail PSU's, the combined 12V rating (in amps) is what matters, and how many 12V rails there are. The individual 12V rail capacities are basically irrelevant unless the PSU label says it has more than 20A available on an individual 12V rail. Also since the only thing that makes them 'split' is current limiters (which do nothing unless they are tripped), there is really NO stability / overclocking advantage in having dual or more 12V rails compared to an equivalent single 12V rail PSU. SilentPCReview explains further.

How to work out the 12V combined amps:

Most newer PSU's should have a wattage listed just below the 12V1 and 12V2 current 'ratings'. For example the antec neo HE 430w, the specs shown in the picture below, has 12V1+12V2+12V3=48A. but look below that in the specs table and the 12V combined rating is 384w, or 32A (384/12=32 - amps is watts / voltage and the voltage is 12V in this case). If the PSU you are looking at doesn't have this information, theres a guide on getting an estimate of total 12V amps from the 'how they work - a more detailed look' section. Alternatively some PSU's list this in the manual, or below the main rail ratings on the PSU label.

High end SLI/Crossfire (x1800xt/x1900xt/7950GX2/8800GTS/8800GTX etc) users make sure you grab a single or triple/quad 12V rail PSU as its been found that most dual 12V rail PSU's won't work at all in such systems.
See the ExtremeTech's article. The dual 12V rail psu's on test, and the Silverstone 650w ZEUS (older revision - problem fixed in the new revision) all shut off with crossfire as they were running both PCIE graphics cards off one 12V rail (both the motherboard power and power through pcie connectors), which caused the PSU to shut down when 20A current limiter was tripped. In the Zeus 650w case a workaround of using a molex-pcie connector can move one of the graphics cards to 12V4 (instead of 12V3), but this can't be done on a dual rail psu, since only the 4pin ATX12V is wired to 12V2 (everything else on 12V1). Some newer dual 12V rail PSU's are just a single 12V rail (such as Antec Earthwatts 500w), or have current limiters set to > 20A, so research before buying. The PSU calculator above has tests to predict whether your system will have issues with dual 12V rail PSU's.

Also look for a PSU that has the connectors that your computer needs. For example newer PCs (most of athlon64 939, P4 775) want 24 pin (or 20+4) PSU's, so don't buy an older (20pin) PSU for them. If your buying a psu for a 20pin motherboard it's often a good idea to buy a 20+4pin PSU so you can use it when you upgrade in the future. Also higher end PCIE graphics cards (eg 7800gt, X800XT) use PCIE power connectors, which many PSU's don't have, so check that your PSU has a PCIE connector (or two in the case of SLI - adapters can be used, but not ideal).

Once you know what basic sort of PSU you need and are looking at individual models, its best to read up on some reviews beforehand. Unfortunately most of the power supply reviewers on the web know very little about PSU's (so often make misguided conclusions and common mistake errors such as using BIOS/software for voltage readings), and often use inappropriate test systems (such as testing an SLI capable PSU on a light, single videocard system). Instead it's better to look for reviewers who load test the PSU to its maximum ratings (or at least test the PSU on a system more powerful than yours) and have a good knowledge of the technical workings of a PSU, such as X-bit labs, jonnyGURU, HardOCP and Silent PC Review. Also be alert for incompatibilities, for example many high efficiency PSU'S (such as Antec Neo HE series, Seasonic S12 500w / 600w - fixed in latest revision for both), have notable minimum load requirements, so are not suitable for very light systems with certain motherboards. The section on 'important PSU issues with popular brands' below has more details on this.

See also a list of known manufacturers of power supplies Here and Here

[edit] Recomended Power Supplies

Note: this is far from an exhaustive list, but more just PSU's that offer top value for money. To work out which category you need first use the psu and watts 12V calculator and then choose a psu in a category at or above whats recommended.

Important: some manafacturers (such as DFI) mandatate a minimum psu spec regardless for their particular motherboard/graphics card etc. So even if the psu calculator puts you well in the clear for say a 400w psu, they may refuse to give technical support because you didn't meet their minimum '500w' or '550w' spec.

Disclaimer: Any change of details for any of the listed power supplies by the manufacturer is not our fault as the details of the listed power supplies are correct at time of posting. This list of power supplies only provides very basic details of each power supply. If you need further information or more in-depth details of a certain power supply or power supplies then you have to research it yourself. In general, this list just provides a very basic comparison between the power supplies.

Posting Note: In future anyone adding recommend PSU's please set out details of power supply in the same format as the ones below, thanks.

ATi/nVidia Links:

• ATi's CrossFire Certified Power Supplies List
• nVidia's SLI Certified Power Supplies List

Note: for the PCIE connectors listing '6' refers to standard 6 pin pcie connectors, '8' refers to the new 8 pin pcie connectors that some high end cards use. In 99% of cases the 8pin connectors can have 2 pins unclipped to be used as a 6pin pcie connector if required.

[edit] Bargain power supplys (under $80)

Manufacturer	Range	Model / Link	Watts	12V Rail(s)	EPS	Modular	A/PFC	Sleeved	PCIe 6/8	Other Features	Review	Price
Antec	Earthwatts	EA 380	380w	27A					1/0		Overclockers	~$66
Antec	Basiq	BP 350	350w	23A					1/0			~$40
Coolermaster	eXtreme Power	RS-430-PCAR	430w	29A					1/0		jonnyGURU	~$46
Coolermaster	eXtreme Power	RS-460-PMSR-A3	460w	29A					1/0	Replaces RS-430	hardwaresecrets	~$55
Silverstone	Strider	ST350	350w	25A					0	Replaces ST365		~$47
Zalman		ZM360B-APS	360w	26A					2/0		Hardware Secrets	~$68
Vantec	ION2	460W	450w	30A					1/1			~$55

[edit] Enthusiast (Under $120)

400w or greater, 28A or greater on 12V rails combined, 1 PCIe connector

Manufacturer	Range	Model / Link	Watts	12V Rail(s)	EPS	Modular	A/PFC	Sleeved	PCIe6/8	Other Features	Review	Price
Antec	TruePower Trio	TP3 430	430w	29A					1/0			~$85
Antec	Earthwatts	EA 430	430w	30A					1/0		SPCR	~$80
Antec	Earthwatts	EA 500	500w	34A					2/0	Actually single 12v rail	jonnyGURU, Hardwaresecrets	~$99
Antec	Earthwatts	EA 500D	500w	34A					2/0	replaces EA-500		~$99
Corsair	VX Series	VX450W	450w	33A					1/0		jonnyGURU, SPCR	~$92
Seasonic	S12 II 430w	PSR430	430w	30A					2/0		SPCR	~$87
SilverStone	Element	ST50EF Plus	500W	35A					2/0		jonnyGURU, SPCR	~$92
SilverStone	Strider	ST56F	560W	36A					2/0		SPCR	~$101
Zalman		ZM460B-APS	460w	34A					2/0		X-bit labs, SPCR, Driverheaven	~$98

[edit] Value high end (under $180)

500w or greater, 35A or greater on the 12V rail(s) combined, 2 PCIe connectors. Independent voltage regulation.

Manufacturer	Range	Model / Link	Watts	12V Rail(s)	EPS	Modular	A/PFC	Sleeved	PCIe6/8	Other Features	Review	Price
Antec	TruePower Trio	TP3 550	550w	42A					2/0	Single 12v rail, not triple	SilentPCReview	~$109
Antec	TruePower Trio	TP3 650	650w	52A					2/0	Single 12v rail, not triple	jonnyGURU	~$140
Corsair	HX Series	CMPSU-520HX	520w	40A					0/2	Single 12v rail, not triple	SPCR, jonnyGURU	~$127
Corsair	VX Series	VX550W	550w	41A					2/0	Single 12v rail, not triple	jonnyGURU	~$127
Corsair	HX Series	CMPSU-620HX	620w	50A					0/2	Single 12v rail, not triple	jonnyGURU, X-bit labs, SPCR, HardOCP	~$143
Seasonic	S12 Energy Plus	SS-550HT	550w	41A					2/0	Very quiet, single 12V rail, not quadruple	SPCR	~$128
Seasonic	M12	M12-500	500W	38A					2/0	Actually single 12V rail		~$127
Thermaltake	ToughPower	W0103	600w	48A					2/0		X-bit labs	~$158

[edit] Premium high end (under $300)

700w or greater, 56A or greater on 12V rail(s) combined, 4 or more PCIe connectors of those at least 2 8pin. Independent voltage regulation.

Manufacturer	Range	Model / Link	Watts	12V Rail(s)	EPS	Modular	A/PFC	Sleeved	PCIe 6/8	Other Features	Review	Price
Antec	CP	CP-850	850w	65A					2/2	Only fits in high end antec cases	jonnyGURU	~$200
Antec	Signature	SG-850	850w	62A					2/2		jonnyGURU, HardOCP	~$280
Coolermaster	Real Power Pro	RS-850-EMBA	850w	70A					2/2		X-bit labs	~$249
PC Power & Cooling	Silencer	750W RED	750w	60A					2/2		Hardwaresecrets, HardOCP	~$249
Seasonic	M12D	M12D-850w	850W	62A					0/4		JonnyGURU	~$270
SilverStone	Olympia	OP1000	1000W	80A					4/2	Older revisions had no 8pin PCIE	HardOCP	~$260
Zalman	ZM	ZM850-HP	850w	60A					2/2		X-bit labs	~$220

[edit] Insane (no price limit)

850w or greater, 65A or greater on 12V rail(s) combined, 6 or more PCIe connectors and of those at least 3 8pin PCIe. For 3 way SLI/Crossfire with extreme cooling.

Manufacturer	Range	Model / Link	Watts	12V Rail(s)	EPS	Modular	A/PFC	Sleeved	PCIe 6/8	Other Features	Review	Price
Corsair	HX	1000	1000w	80A					0/6		HardOCP	~$330
Enermax	Revolution	850	850w	70A					0/6		JonnyGURU XbitLabs	~$320
Enermax	Revolution	1050	1050w	87A					0/6		HardOCP	~$350

[edit] Quiet Power supplies

These power supplies have proven to be the some of the quietest avaliable, generally as confirmed by a SilentPCReview or equivelent review. PSU fan speed under typical load should be < 850rpm for 140/135mm/120mm fans, and < 1200rpm for 80mm fans. This is far from an exhaustive list, most corsair and seasonic brands for example could be put in this category (except the > 700w models that are louder).

Manufacturer	Range	Model / Link	Watts	12V Rail(s)	EPS	Modular	A/PFC	Sleeved	PCIe 6/8	Recommended for:	Review	Price
Coolermaster	eXtreme Power	RS-430-PCAR	430w	29A					1	Budget silencers	jonnyGURU	~$46
Enermax	Modu82+	525	525w	50A					0/3!	Extreme silence	X-bit labs, Anandtech	~$115
Enermax	Modu82+	625	625w	50A					0/4!	Extreme silence	SPCR	~$161
Coolermaster	Real Power Pro	RS-850-EMBA	850w	70A					4/2	Quad SLI overclockers	X-bit labs	~$249
Zalman	ZM	ZM850-HP	850w	60A					4/2	Quad SLI overclockers	X-bit labs	~$220

[edit] Top Value Case and PSU Combos

Here is a list of cases that come with decent psu's (ie good brand, 24/20+4pin and pcie connectors, decent 12V rail(s), unlike most of the $40-$80 cases that have poor quality generic psu's in them. Avoid all Thermaltake bundled case + psu combos for reasons listed in Thermaltake_PSUs, unless you only ever plan to run light or older systems, since they are deceptively lableled (rated for 250w by oem which thermaltake then sticks a '400w' or '430w' label on it), and are based on the obselete atx1.3 standard so are not designed to run modern systems.

Manufacturer	Range	Model / Link	PSU Model	Watts	12V Rail(s)	PCIe	Case Type	Review	Price
Asus		TA-582	Bespec	350w	25A	1	mid tower case		~$70
Antec	Solution	NSK2400	SU-380W	380w	28A	1	HTPC suited desktop case	SilentPCReview	~$140
Antec	Solution	NSK4480/b	EA-380W	380w	27A	1	Small mid tower (fits full size atx board)		~$101
Antec	Solution	NSK6500	EA-430W	430w	32A	1	Larger midi tower		~$140
Antec	Sonata	Sonata 3	EA-500D	500w	34A	2	Larger midi tower		~$175

[edit] Trusted Power Supply Makers

Here is a list of user supplied information from the forums.

Along the lines of "trusted" PSU's (considered brand name) are:

• Antec (not older models - see below)
• AOpen
• Channel Well Technology (OEM for Antec, some Corsair, Foxconn, some Enermax, Xclio, and Thermaltake ToughPower series)
• Coolermaster
• Corsair
• Delta (OEM for Antec, Dell)
• Enermax
• Enhance (OEM for some OEM for Antec, SilverStone, some Zalman)
• Fortron Source / FSP (OEM for AOpen, PC Power and Cooling, OCZ GameXStream series, Sparkle, Zippy, Zalman)
• Foxconn
• Great Wall / Wide Tech - Suprisingly decent power supplies despite the price tag. Sold as Great Wall, GW, and WideTech.
• Hiper
• OCZ
• PC Power and Cooling
• Shuttle (made by SilenX)
• Seasonic (OEM for Antec Neo HE, Corsair)
• SilverStone
• Sparkle Power
• Super Flower / TTGI
• SunBeam Tech (NUUO 550w only)
• Tagan
• Thermaltake (high end ToughPower units only, avoid the bundled case + psu combos including TR2 430w, 400w, 480w Butterfly for anything but old PIII/Athlon XP/light newer systems). See Thermaltake PSUs For more information.
• Topower / E-Power (Also OEM for OCZ Modstream / Powerstream, Tagan, some Enermax, Super Flower)
• Vantec
• Ultra (certain models only - see below)
• XClio (USA mainly)
• Zalman
• Zippy / EMACS

[edit] Some Not Recommended Power Supply Makers

Some of these may not seem dodgy and some may be because of numerous "bad" reports, but these power supplies are almost definitely of questionable quality or have low specifications (and/or have far inflated wattage ratings / rail current ratings of what the power supply can actually do under normal conditions). Just because you can get away with one of these power supplies does not mean the next guy will, or the guy after that, whatever.

• Aspire
• Auriga - AVOID AT ALL COSTS. Might work today, or tomorrow, or the next month, BUT down the track you'll be unable to turn them on. So far had a 50% return rate on them after 3 years approx use. Expecting the other 50% to fail in time as well.
• Clipper pro
• Codegen - SUBJECTIVE, I've got one 400W working like 16h/day for 6 years and still running. Plus I've seen about 50 cheapies of this brand gone and hasn't come back for warranty. They're the only brand offering 5 YEARS warranty on them. Main issue is the vast majority of models are all old designs, OK for P3/athlonxp days but poor voltage stability on newer systems, which is unacceptable given ATX v2.0 has been around for years and they still haven't got any 'proper' ATX 2.0 psu's with decent 12V specs (only rebadged old designs that are deceptively labeled). example (550w psu, 18A 12V rail)
• Coolmax
• Generic (duh)
• Hairong
• Honli
• Logisys
• Mad Dog
• MGE / XGbox
• Omni - Stay away from their cheap product lines. But their mid-end one is OK though (e.g. 500W silent, 12cm fan, honey comb vent).
• Powmax
• RexPower
• Ritmo
• SHAW
• TT (Not to be confused with ThermalTake) - These babies blow capacitors like no other!!
• YoungYear (OEM for MGE / XGbox and Aspire)

[edit] Important issues with popular PSU brands

• Antec: The first few revisions (before A3) of the neo HE series had lots of incompatibility issues with various motherboards, Asus in particular. Particularly the 430w model seems to be having issues even in latest revisions, so its probably best to avoid it altogether. rev A4 and above should be fine mostly. Also the smartpower series is having high return/fault rates currently, so is not recommended. Finally both the smartpower and truepower 1.0/2.0 (3.0 is fine) series are all full of Fuhjyyu caps, so often die after as little as 6 months in a warm case. All other Antec psu's should be fine however, the newer models all have at least passable capacitors.
• Coolermaster: their psu's are sourced of good oem's, but some dubious decisions are made in marketing their psu's. In particular the xtremepower 600w (Johnnyguru review) is based on a 500w Seventeam unit (so therefore can't do the full 600w advertisied). Apparantly Coolermaster marketing said they 'tweaked' the rails to get the extra 100W wattage and extra 6A 12V rating, but from johnyguru's review its obvious that past 500W (what the oem rated it for), the psu unusable. Their 550w 'real power' model is better though, actually does its rated wattage rating and considerably more than the rated total 12V rating Johnnyguru review
• Enermax: Some of their older models like the liberty series used some cheap capacitors that failed . Newer infiniti/modu series etc are better with all japanese capacitors. Currently modu82+/Pro82+ are having quite a few DOA units, so won't be recommended until they're reported as better.
• FSP: Avoid any epsilon based power supply above 500w. They are all group regulated designs with increasingly worse voltage ripple and regulation issues. The design is only really good for 600w of clean power, yet is avaliable in models as high as 1010w.
• OCZ: Avoid the ocz modstream for anything but light to medium single videocard systems due to huge cross loading requirements for anything past about 15-20A load on 12V rail (psu starts to overvolts the 5V rail and undervolts the 12V rail). Also the OCZ gamexstream/silentxstream series above 500w suffer from ripple issues, and somewhat poor voltage regulation, as they are FSP epsilon builds.
• SilenX: Are also Fortron rebrands (some of them any way), but are overpriced for what they are.
• Silverstone: The older revisions 650w ZEUS and ST60F psu puts both pcie power connectors on the same 12V rail (fixed in latest revision), so will have same problems as dual rail psu's with high end SLI (workaround is to use molex-pcie adapter for 2nd videocard, then all goes well). Also the strider series (360w/400w/460w) still don't have any PCIE connectors. Otherwise they are generally high quality psu's.
• Sintek: Their new power supplies are possibly made by youngyear, this is yet to be confirmed.
• Superflower/TTGI: Moved down because of some possible Quality Control Issues in later Power Supplies. This will remain here until the Q.C. Issues are fixed (If they are fixed). Older models shouldn't be greatly affected, if you have a Super-Flower, don't ditch it.
• Thermaltake: Their PSU's are of reasonable to good quality, but their lower end is deceptively labelled (eg xp480 400w model is rated for 250w by the oem), and is based on the obselete atx1.3 standard with a small 12V rail. The rest of the range is a mix, with some being excellent (Toughpower series), others are terrible to good. See Thermaltake_PSUs For more information.
• Ultra: Any ultra X-connect PSU from Xmas 2004 onwards is most likely a piece of crap. Their newer infinity and X2 series are ok for medium sized systems, nothing special (different OEM). The new X-Pro series are excellent.

[edit] How They Work - A More Detailed Look

Power supplies have 3 Main rails, the +3.3v, +5v and +12v rails. Each of these rails has an current rating (written in xxA, eg 25A).
Remember that:

Current * Voltage = Power (Watts)

So , for example, if the 12v rail has 26 amps, it can produce 312w on that rail alone; the same as the others. There are other rails (eg -12v : used for serial port, +5vsb : used to power motherboard while computer is turned off), but I wont talk about them here.In today’s computers, CPU's and Video Cards draw their power from the 12v rail, so you obviously want a PSU with a higher current on the 12v Rail(s). For example X-bit labs measured an x1950xt to draw ~118W off the 12V rail (~95% of total), and ~6.5W off the 3.3V rail . Use the PSU calculator linked above to get a guide on how much 12V amps you need.

Some newer power supplies have 2 or more 12v Rails. This was introduced to comply with EN/UL 60950 safety regulations of no more than 240VA (20A) in any one wire. There was the suggestion to also to give the CPU a completely independent rail for cleaner voltage, however for economic reasons the vast majority of PSU's don't have fully independent (i.e. separately regulated) 12V rails. New dual transformer psu's such as Enermax Galaxy, Tagan dual engine are an exception having 2 (but no more) independent 12V rails, though some of these join the 12V outputs instead. So far all PSU's that have been examined measure a 'short' between 12V2 and 12V1. In most cases, one rail (12V2) is dedicated entirely to the CPU, and the other rail (12V1) runs all the other components (video card(s), hard drives etc). Increasingly more power supplies (especially EPS) are coming out with 3 or even 4 rails, to handle computers that need more than 20A 12V capacity for components (such as crossfire x1900xt systems), and/or multiple cpu systems. A more detailed discussion of the issue can be found at SilentPCReview.

On this topic, when you see a power supply which has for example 2 12v rails rated at 18A, don't immediately think it has 36A. That's how much each rail can produce individually (in actual case most will do close to 20A on an individual 12V rail, sometimes more), the combined individually current ratings is not always this. For example, the Enermax 535W FMA has two 18A 12V rails, but also specifies the combined 12V as 34A. The reason for this is that both 12V rails essentially come from the same source (transformer tap and regulator / filters), but are then split off into two rails, both current limited to 20A. So a less than honest power supply maker could actually say their 500W 'yumcha' has 2 18A 12V rails, yet only can do 22A combined (and does not specify this limitation on the specifications). Furthermore, dual rails are gradually being phased out, as SilentPC Review's excellent guide to dual rails reports, since Intel has removed it as a requirement for PSU's, so Dual 12V rail's may disappear altogether in the future. Furthermore the above link mention that there are many PSU's already for sale that despite being labelled as having dual rails, are only a single 12V rail (with no 20A current limiters). Examples include most Seasonic made PSU's. In particular from Intel's PSU validation results, many of the PSU's that failed the 240VA do not have dual 12V rails at all, perhaps half.

Since the combined 12V rail capacity isn’t always specified, to get an indication of the combined current rating, take the PSU’s wattage (NOT including the -12v/+5vsb/whatever other rails, these usually add up to about 20w), subtract from it the combined max of the 3.3v and 5v rail (this is almost always listed), and divide whats left over by 12.

If that was too complicated, here's an example, lets take the Antec Smartpower 2.0 500.

As you can see, the max for the 3.3 and 5v rails is 180w, the max for the 12v, 5v, and 3.3v rails is 480w. So 480-180=300. You will get AT LEAST 300w from the 12v rails combined, or 25A, note, at least, you will probably get more, depending on how many hard drives/optical drives you have. This will almost always (except for perhaps some deviously labelled generic PSU's) underestimate the available 12V capacity since on any decent PSU, the rail ratings usually add up to far more than the wattage ratings. For example the Antec NEO HE 380w has the following ratings:

12V = 336w, 5V = 60w, 3.3V = 70W, -12V = 7W, 5VSB = 12.5W. But 336+60+70+7+12.5 = 486W! 

The reason for is is that a PSU's wattage rating is determined primarily on heat/thermal issues (heat created due to combined losses of all the rails in the psu - remember that no PSU is 100% efficient and will always produce a some amount of heat when providing power to the PC), while individual rail ratings are based on the current capacities for the regulators/components for that rail. In short if the PSU's rails actually do add up to the wattage rating (or in some cases even fall short of), its generally a sign that you can't trust the ratings of that PSU (so avoid it).

On a special note about over clocking with PSU's with multiple 12v rails, (Intel P4s), a heavily overclocked P4 dual core can pull 200w+ peak, which could trip the 18-20A current limiter on 12V2 rail (causing PSU to shut down). An additional problem here is that a few dual rail PSUs also power one of the videocards off 12V2, which works fine for athlon64 (and has benefit of being able to better utilize 12V capacity), however the combination of the overclocked videocard (eg 7800gtx) and the higher consumption of an Intel CPU can quite easily exceed 240W even with moderate overclocking. So if you are over clocking a P4 especially the dual core variants (and I mean REALLY over clocking it), go for a psu which has one giant 12v (eg 33A+) instead of one with two smaller 12v rails (eg 18A+18A), or a quad 12V rail psu with an EPS connector and motherboard with an EPS connector to match(this feeds cpu power off seperate 2 12V rails). Some examples of these kinds of PSU's are the OCZ Powerstream 520w (rebrand of the topower 526p6 and 626p6) and the Antec Truepower 550w EPS (Only the EPS one), Seasonic S12+ / Corsair HX series. Also dual rails can also pose a problem for high end SLI setups. The problem is that with 2 high end graphics cards (up to 10A each) and multiple hard drives, the user can exceed the 18-20A current limiter for 12V1 rail. In such cases a single rail, or triple/quadruple rail is needed. Also for these extreme systems it would be wise to get a psu that has been found to pass low 5V / high 12V load tests (for Johnnyguru, X-bit labs test this). The reason is that these systems have such high levels of 12V load, but very little 5V/3.3V load, the 12V rail can often sag (Eg Tagan easycon 530w - as jonnyGURU shows. The Xbit Labs PSU Guide has more detail on this.

NOTE: Some Athlon XP's (motherboards without ATX12V/P4 connector), PIII's, and CPUs before that drew their power off the 5v rail, so you'd want about 30A+ on the 5v Rail if running and over clocking an Athlon XP (and possibly more for heavy overclocking and if graphics card loads 5V rail heavily as well).

Further reading can be found here

This portion was written by numerours OCAU members including SnooP-Wiggles