Welcome to mpilchfamily's College of PSU Knowledge. 
All information found below was compiled from the links listed in the reference section. Changes have been made in an atempt to make things easier to understand.
This article has been broken down into two sections.
In PSU 101 we will cover some of the basics about PSUs that you need to know. We will go over some basic terms used with PSUs that you need to know. Then we will discuss the PSU's Label and how to find the important information you will need to know about any PSU you may be looking at. Then we will cover some information about finding the power needs of your system so you can find a PSU that best fits your system's needs.
In PSU 102 we will cover several of the common features found on PSUs. We will cover the pros and cons of each and hopefully dispel any misconceptions about these features.
PSU 101: Things to know when choosing a PSU.
The Power Supply Units (PSU) is the most overlooked component in a PC. Lock ups, artifacts, and an inability to over clock can be attributed to low voltages and inadequate current. Yet many people overlook their importance when considering a new build. My hope is this article will educate you on what you need to know when considering a PSU.
Before we get started we need to cover some preliminary terms...
Amps, Amperage or "A":
The strength of electrical current is measured in Amps. Think of it like the current of water running down a river.
Volts and rails:
A volt is a standard unit of electrical potential. PSUs supply several voltages to your PC. +3.3V, +5V, +12V, -12V and sometimes -5V.
Watts:
Watts is a standard unit of measurement derived from taking Amps and multiplying by Volts. That said you can probably see how knowing the "wattage" of a power supply may not be enough since you have multiple rails, delivering different voltages, capable of different amperages and the total wattage is really just the sum of the capability of all of these rails.
PSU label:
The way most of you will be getting the information about a PSU is from the label. The specs on the label are important beyond what the total "wattage" written on the box. You need to be aware of the amount of current your system needs. You also need to be aware of the total current output of the PSU on the +12v rails. Now the total amperage on the combined +12v rails is
not the sum(Addition) of the rails. You need to look at the Max wattage being supplied to the +12v Rails. The reason for this is the amps listed per rail are the max amount of current that rail can handle without failing. It is not the max current being provided to the rail.
To calculate the max amount of Amps we use the following equation, Watts divided by Voltage equals Amps (W÷V=A). The true electronic notation for each is different but to keep it easy to understand I simplified it to the first letter of each word. For those who are interested thou here is the typical breakdown
Watts = P
Volts = E
Current/ Amps = I
Anyway, this will give you the actual total current output of the +12v rails. Most systems today with a midrange CPU and Video card need between 20-26 amps across the +12v rails. With the Release of the new DX10 compatible cards the power requirements have jumped. Most systems without a video card pull about 14A-16A. The new 8800 series from NVIDIA pulls 14A-16A on its own.
Most PSUs don't mention on the label how much power is on the combined +12v rail(s). Though many top quality PSU will have at least the max +12v wattage listed and some even tell you what the combined amperage of the +12v rails are. Many will mention the max power on the +3.3v and +5v rails. Though it may seam subtracting this number from the total wattage would give you the +12v's max wattage. This is not the case. If the label doesn't mention the total wattage being supplied to the +12v rails then you have no way of calculating it out. While there are many PSU that don't have this information listed on the label they sometimes have it listed on the manufactures web page.
Here is a good example of a well laid out label:
Notices listed under the +12v rails is the max wattage of the rails. Dividing this number by 12 gives us the total amperage. You will also notice that Enermax is nice enough to already list the total amperage next to that wattage. Some labels may look like this and just mention the total amperage without mentioning the wattage.
Other labels may list the total +12v wattage/ amperage like on this Antec PSU.
About 90% of the systems power usage comes from the +12v rail(s). This is why we focus so much attention on it and not the +3.3V and +5V rails.
Be aware that the numbers on the label are only as good as the company who print them. For example you could have 500W PSU with Dual +12v Rails. One the +12v1 rail you have 14A and on the +12v2 rail you have 12A. But the max output is only 180W on the +12V rails. Using our formula from above, we find the actual total current output is only 15A. At the same time you could have a PSU with the same amount of amps on the +12v rails but gives you 264w of max output. This gives you a total of 22A on the +12v rails. Be aware of what you are getting when buying a PSU. Always check the label and check the numbers. Don't choose a PSU because it has a lot of power and the price is almost too good to be true. You will also want to stick with a good name brand PSU. For a list of PSU in order of quality see:
xtremesystems' PSU Ranking
jonnyguru's PSU Listing.
Next you need to know how to determine the Watts you need to run your PC.
Calculating Current:
Many people overlook the importance amperage has in regards to your PC. You can find all sorts of PSU on the market that supply more then enough watts to run your system but lack the amps it needs to back those watts up. It's like being all talk and no action. So it's important to know how many amps your system needs to run.
Very few components in a PC will have a listing of how much current they draw from the PSU. The easiest way I've found to calculate the total current your system will need is buy using the
eXtreme Power Supply Calculator. There are now 2 versions of this Calculator. The Lite version is the one that's been available for free. They have now added a Pro Version that requires a subscription. The Pro version is very useful and gives you accurate amperages your system will need from 3.3V, 5V and 12V rails. I have found this added information is well worth the price.
It would be a good idea to support the Pro Version of the Calculator. It wouldn't surprise me at all if they got rid of the Lite version all together. It's a very helpful tool and you know you are getting as close to an accurate figure as you can get. Notice that it gives you the figures as if the system was under a 100% load. Not that you will ever achieve that kind of a load under real world conditions but it’s a good solid figure to go by.
For those cheap skates out there here is a method for using the light version to find a ruff estimate of your system's power needs on the +12v rails. Not all of your PC components need to be entered into the calculator to get the max current your PC will use on the +12v rail. Only the CPU(s), Video card(s), 1 HDD (2 or more if you're running a raid configuration) and 1 Optical drive will be running at close to 100% at the same time. So these items are all you need to enter into the calculator to get your number. If you OC, plan to OC, and/or run water-cooling be sure to enter that information into the calculator as well. Yes the fans, and other lights will also be running near 100% but they account for very little in the overall usage of the PSU's power on the +12v rail(s). Unless you have a lot of lights or fans in your system then you may want to add these to the list just to be safe.
Now we use the same amps calculation from above to find what we need. So take the wattage you just got from the calculator and divide that by 12 to get your total current needed. Personally I like to round up a bit on the number just to be on the safe side. You might also want to consider any future upgrades you may have. Input those into the calculator. It's good to go ahead and get a PSU that will fit your future needs. The amperage you get using this method is a bit over inflated. Many of those items like the Drives use a bit of the 3.3V and 5V rails as well. Though most of the power comes from the 12V rails.
If you're looking to get a ruff idea of an individual part's power needs then just enter the single part into the calc. Now subtract the 38W the calc gives you for the motherboard and divide that by 12 to get that components +12v amperage usage. This can only be done with the above-mentioned components that use the +12v rail. Once again this will be an inflated number since not all the power that component uses comes form the +12v rails.
Keep in mind that having a PSU that exceeds your system's needs doesn't hurt anything. The PSU will only provide the power and current the system needs. Find yourself a balance. You want to have a big enough PSU that can support any future upgrades you may want. At the same time you don't want to be spending a lot of money on a PSU you don't need.
Conclusion:
As you have seen we have only been talking about the +12v rail(s). As mentioned before 90% of PC components get their power form the +12v rail(s). So be aware of the basics and choose your PSU well. Try to choose a PSU that will fit your needs and leaves a little room for future upgrades you may want. There is no need to go too overboard with your selection. There is really no need to get a 1kw PSU. At least not for now. :wink:
PSU 102: PSU features explained.
Now that you have read the PSU 101 portion and have a pretty good understanding about how to read the label, Identify your system's power needs and find a good PSU to suit those power needs. Its time to look into some of the more in depth features of the PSU There are several features that PSUs come with that can play a big role in your PSU decision. Features such as:
- Single vs. Multiple +12v rails
- Efficiency
- Power Factor Correction (PFC)
- SLI and Crossfire Certification
- Modular Cabling
- Cables and Connectors
Single vs. Multiple +12v Rails
Another heavily debated topic is weather a PSU with one strong amperage +12v rail is better then one with multiple +12v rails. In all, the PSUs with single high amperage +12v rail are considered to be the best PSU available and are often among the most expensive for their power range. This I due to the cost of the parts required to have such large amperage on a single rail.
Now most PSUs you will find have multiple +12v rails. These are not true multiple rails. In most cases they are multiple current limited 12-volt rails derived from single rail. This kind of PSU only has one set of circuitry inside the PSU, which generates 12 volts. But it is split into separate 12-volt outputs each of which has their own current limit circuitry. If any one of the 12-volt outputs exceeds its current limit then the PSU shuts down. For example you could have a dual rail supply, which has a single internal 12-volt rail, which can deliver 30 amps. Then inside the PSU its split into two separate rails each of which has an 18-amp limit. If you try to draw more than 18 amps from either of the 12-volt rails then the PSU with shut down. If you try to draw more than 30 amps of total current from both of the rails then it will also shut down (assuming that the internal 12 volt rail also has a current limiter).
This kind of PSU exists because of safety standards. The
IED 60950 standard limits wiring to 240 VA (volt amps). At 12 volts that means that a wire is only allowed to carry a maximum of 20 amps. The standard exists to try to limit the amount of current, which flows in a short circuit before the PSU shuts down. That can reduce the likelihood that a short will cause a fire or destroy something. So if your PSU needs to deliver more than 20 amps at 12 volts and obey the safety standard then it needs to have more than one 12-volt rail.
While a PSU with a large, single +12v rail can transfer 100% of the 12-volt output from the PSU. A multi-rail 12-volt design has distribution losses of up to 30% of the power supply's rating. Let's use this 700W OCZ PSU as an example since it lists what each +12v rail powers.
PSU Label Picture
As you can see the 1st +12v rail powers the CPU, 2nd is for the PCI-e2/ CPU 2 connectors, 3rd for the Motherboard and other peripherals and 4th is for is for PCI-e1. Now each rail has a max of 18A it can handle without failing. The PSU is able to supply about 42A to 50A on the combined +12v rails. It's a little hard to get exact numbers on this due to the somewhat vague label. For the sake of argument we will call it at 45A. So let's get an idea of what kind of loads we can expect per rail.
Well an average mid-range CPU will pull about 7A to 9A so there you can see a lost in power already. If you're running dual video cards or a single high-end card, that takes both of the PCI-e connectors, then there is at most 6A being drawn from each of the PCI-e connectors. So the 2nd and 4th rail only have a 6A draw. On an average system there would only be about 12A to 16A being drawn from the 3rd rail. So we are looking at a total system draw of about 39A. In order for you to even get close to the 45A total you would have to be OCing the hell out of the CPU or perform a volt mod to force more then 6A to be drawn from each of the PCI-e connectors.
So as you can see multiple rails don't allow you to use the PSU to its full potential. This is why considering the number of +12v rails and the distribution of the system's power across those rails is important. It would be fairly easy to overload the 3rd rail if you had several drives and other extras in your system like a tuner card. Since the maximum current from any one 12-volt rail of a multiple-rail PSU is limited to 20 amps (240VA / 12 volts = 20 amps), PCs with high-performance components that draw over 20 amps from the same rail are subject to over-current shutdowns. A strong amperage single railed PSU like PC Power & Cooling's 750W Quad, which has a max of 60A on it's +12v rail, would be a better option since all component draw there power from that single rail. None of the power gets lost among current limited rails. Overall the Single rail design is more efficient in its distribution of power.
Efficiency
The efficiency rating of a PSU tells you what percentage of the power being drawn from the wall is actually being converted over to usable power for your PC. You will find that on average most PSUs are between 70% and 85% efficient. Real world figures are quite different and very depending on operating temperature and load. Most quality PSUs should average about 70% or better. For this discussion we will be using 80%.
So why is this information so important? Well if you're not the one paying the power bill every month or so then you are probably not that concerned. But a less efficient PSU contributes more heat to your system. SO for that aspect it's a good idea to have an efficient unit.
When a PSU converts the AC power from the wall into DC power your PC can use there is always energy lost in the presses to heat. This is a limitation that may never be overcome but we will get as close to 100% efficient as we possibly can. Anyway, so if we had a 500W PSU under a 100% load and is 80% efficient then it would be drawing about 600W from the wall. A 500W PSU will provide a full 500W of power and doesn't have a lost of power because of the efficiency. The less efficient the PSU is the most power it has to pull from the wall in order to reach its full potential. Now under real world circumstances you will not be running your PSU at 100%. Those of you with 600W or larger PSUs don't worry your not pulling that much power from the wall. Most PCs average about 250W to 300W of power draw. Those higher end systems with SLI/CF and OCing can draw quite a bit more. But on average most systems don't exceed 300W. Now let's try not to argue over the average systems power usage. For this article the average system pulls no more then 300W. So under everyday usage your system averages 300W from the PSU so you are pulling about 360W from the wall.
Now we will go a little more in depth on the subject. As mentioned before the efficiency of the PSU will very depending on temperature and load. The efficiency range actually follows a bell curve. PSUs are at there most efficient when under a mid to high load.
X-bit labs
As you can see by this example there is a curve and as a larger load is put on the unit the more efficient it is. So it's best to have a unit that is close to your power needs and doesn't surpass it by a great deal. You wouldn't want to have a 1000W unit powering your system when all you need is about 450W to 500W. That 1000W unit may not even reach 60% efficiency with such a small load on it but the 450W to 500W unit would be able to almost reach its full efficiency of about 80% to 85%.
So now you can see if you have a PSU that averages about 70% efficiency how this can affect your electric bill. Like wise the less efficient the unit is the more heat it generates because it can't convert as much of the AC power into usable DC. So instead of pulling 360W from the wall a 70% efficient PSU is pulling 390W. These are all in Watt Hours. So with a less efficient PSU you are adding another 30W an hour to you bill. It can add up pretty quick. You have also learned that an oversized PSU can be less efficient then one that more closely meets your system's power needs. Just another reason why you shouldn't go over board on your PSU selection. You can now find PSUs marked with an 80 Plus emblem. This indicates that the PSU meets the standards and has been certified 80% efficient.
Power Factor Correct (PFC)
In order to better explain this we need to first understand what Power Factor is. The "Power Factor" of a device or network is defined as the ratio of "apparent power" and "real power"; PF=(apparent power/real power)
Real power is the power dissipated when the voltage and current are in phase. Note all the power is "positive" (above the zero line)
where "e"=ac voltage, "i" = ac current and "p" = ac power
A complex device such as a power supply contains components that cause reactance (inductors and capacitors) and that is reflected on the AC network "fingerprint" as a phase shift (seen as θ below) between the voltage and current. Note some of the power dips below the zero line, this is the "complex power" component and causes "apparent power" to be dissipated.
This angle Į can be expressed as the tip-to-tail vector sum of the real and complex components, the result being "apparent power". Their relationship is better visualized in the "power triangle" below:
to calculate the power factor, PF=(real power/apparent power) = cos θ
So why is this bad? When the apparent power becomes dominant the power company has to supply a much greater power requirement over the real power for an equivalent load. An example, a UPS unit is rated in VA since it has to provide both real and apparent power to the PC it is powering, much similar to the utility providing power to your house. A PF corrected power supply will go a bit further on UPS power than one that does not have any PF correction. On an industrial / corporate scale, the sum of multiple low PF devices can give a poor power waveform and harmonics in your power feed.
So PFC, In simple terms this is creating a reactance across the power supply input that is equal and opposite to the reactance that is intrinsic to the device, so the outlet "sees" less apparent power and more real power. In a nutshell, power factor correction is making a "complex" device "simple" to the perspective AC power network:
Active power factor correction is a circuit that monitors the load across the input and will vary the opposing reactance accordingly, so the angle Į is kept to an absolute minimum as the load of the PC varies.
Passive power factor correction is a simple, hard-wired circuit that provides a fixed opposing reactance.
Links:
PFC Decoded
Power, Crest, and Surge Factors Explained
PFC Concepts
SLI and Crossfire Certification
You will find many PSUs today carrying an SLI or Crossfire (CF) certification. Some even have both certifications. Companies will even go as far as to have 2 different model numbers for the same PSU. Each model number referring the PSU as having one certification or the other.
There are many PSUs available that can easily support a CF or SLI system but lack the certification. The reason for this is the manufacture has to submit the PSU for review by NVIDIA and AMD/ATI for the certification. For the most part if a PSU is certified for one it will work on the other.
Its important to note that just because the PSU carries the certification doesn't mean it will power all SLI and CF configurations. When it's certified it's certified for a certain range of configurations. So be sure to check NVIDIA's and AMD/ATI's web page for a listing of certified PSUs and what configurations they are Certified for.
Here are the links to each company's certification web page.
SLI Certified PSU listing
Crossfire Certified PSU listing
Modular Cabling
You will find many PSUs with modular cables. These are cables that can be detached from the unit. This helps to prevent clutter inside your case and promotes better airflow. There are several types of modular connections used. Some are better then others and some make better connections then others. If you get a unit with modular cables then make sure they are tight connections. Any loose connections can cause major problems down the road.
Now there are very large debates over the pros and cons of modular cables and weather or not it's worth having. PC Power & Cooling has this to say about Modular Cables.
Quote:
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Originally Posted by PC Power & Cooling
Due to their look, convenience, and cost savings for manufacturers, modular plugs have become a popular power supply feature. Unfortunately, there has been little or no discussion of the impact of this feature on overall performance and reliability. The fact is, modular plugs limit power by adding to electrical resistance. The voltage drop can be as much as would occur in 2 feet of standard wire. Worse yet, modular plugs utilize delicate pins that can easily loosen, corrode, and burn, creating the potential for a major system failure. That's why professional system builders specify uninterrupted wire!
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Power Supply Myths Exposed!
Now while I believe PC Power & Cooling make some of the best PSUs available I do disagree with their statement. But this isn't about my opinions it's about the facts. All connections made in the system add some resistance. Yes there is more resistance in modular PSUs. Now manufactures of Modular PSUs use sturdy connectors and some even use gold plating to help prevent corrosion. As mentioned before you want to make sure the connections are secure. This is to prevent movement in the connection that causes arching of the voltage, which causes power spikes and corrosion in the connection. Some manufactures use a larger gauge wire to help minimize the resistance. Many manufactures say the resistance and voltage drop is negligible or not measurable and will not affect your system. As mentioned before this has become a highly debated topic.
Cables and Connectors
There is an article that covers this topic much better then I can. This article also goes into detail about other aspects of PSUs so it may be a good idea to read through the whole thing.
PSU Cables and Connectors
There has been a fairly recent update in the form of the new PCI-e 2.0 spec which among other things adds a new video card power connector to the PSU. You will now find PSU that not only have the usual 6pin PCI-e connector but also a new 8pin version. Upcoming video cards will use these new 8 pin connectors and some may use one of each. You will find a few of these new units listed in the
PSU Reference List.
Conclusion
Now we have highlighted a few of the key features found in today's PSUs. I hope this has helped to clear up any misconceptions you may have had about these features. Many of these features go hand in hand in how well a PSU can handle a high-end system. Low efficiency, poor use of multiple rails coupled with modular cabling can all combine together to make for a PSU that doesn't live up to it's full potential.
I want to thank all those who have contributed to this article. I don't quite know it all so it's nice to have some help when compiling a resource such as this.
Questions
Just to make things clear all the information presented has been taken from one or more of the resorces listed below. If the information you are looking for isn't here then you should be able to find it in one of the links below. If there are any questions about the above information please feel free to post them here or PM me at any time. If there are any changes or additions you think should be made to this thread also let me know and it shall be considered.
Resources:
PSU Buyers Guide and Reference List
Power, Crest, and Surge Factors Explained
Johnyguru's Tiered PSU List
PSU Myths Exposed
PSU Rankings. See what PSUs are rated the best.
Power Supplies 101: A comprehensive guide
eXtreme Power Supply Calculator Go with the Pro Version
Power Supply Fundamentals & Recommendations
Power Distribution within Six PCs
JonnyGURU Loads of good PSU links here.
PSU Cables and Connectors
Thanks:
Special thanks to all of those who have contributed to the evolution of this article over the past 10 months. Thanks to all those who where on Tom's Hardware Forum durring the compiling of the PSU 102 section. Your contabution to the article was great. A very specail thanks goes out to JonnyGuru. Most everyhting i know about PSUs has come from him and his site. I continue to learn from him on a daily basis.
Thank you everyone.
PSU Selection Guide (PSU 101-102)
