Computer Power Supplies - A Guide

This Guide
This is a guide intended for anyone who would like to learn enough about computer power supplies to be able to choose one for themselves or others. The language will be as simple as possible but there is a Glossary.

Why?
There are many factors that must be considered when selecting a Power Supply Unit, or PSU for short. You may need one for a new system, or simply because you have heard about the dangers of owning a poor PSU. If you are reading this you probably understand that you want a safe PSU, one that won't fail and damage your components in the process. What you may not understand is the difficulty of diagnosing a PSU that has partly failed.

A poor or slightly damaged PSU can cause a system to display a wide range of symptoms which also resemble other issues. It can be almost impossible to diagnose a PSU problem at home without having another PSU to swap into a build. This is a very strong argument for getting a quality PSU... you need to have as much confidence as possible in this part because of the difficulties caused by a bad one.

What makes a good one?
There are several things to look for in a power supply, but the primary concern is the quality of power it produces. The power from your wall is way too energetic for delicate electronics, and that is why all of your electronics have some sort of power supply that converts the wall power to something useable. This is easy if you are powering one little device that has set needs. It gets a lot harder when you are powering a hungry device like a gaming computer that has very different power needs under variable circumstances.
So, your PSU has to be able to produce clean, adequate power while idling, and has to do the same under full load. But then it also has to do so at any level of power demand in its range.
When we say "clean and adequate power" we mean several things, and it starts to get technical. We can easily see one thing though, and that is that making a device that can supply 45W whenever asked is a lot easier than making a device that can supply 50W to 1200W and any number in between.

If you would like to learn more about power supplies and what makes a good one, here are some links. Be warned, it's not something you'll pick up in 10 minutes.
Hardwaresecrets.com - Everything You Need to Know About Power Supplies
jonnyguru.com - Power Supply FAQ
TechSupportForum.com - Power supply information and selection
A list of all the ATX spec sheets
What wattage?
The total wattage of the PSU is not really the most important. Computer Power Supplies have several different voltages they produce, but the most important one is 12V. That's the type of power that you need most to power a modern computer. So, getting a "750W" power supply that only actually produces 600W of 12V power might not work out for you.
Please see the next post for a detailed description of how to read a PSU label. This will help you figure out how much actual 12V power that PSU can supply.
Almost all quality power supplies are capable of producing MOST of their rated wattage as 12V power. There are a few notable exceptions though.

Calculating total power needs
Here are a few links to help you determine how much power you will NEED in your computer:
Atomic MPC forum - Mark84's extensive graph showing power usage of almost any gaming GPU from the last several years.
As the GPU is the largest single load on your PSU, this table can take you a long ways in determining your needs.
Extreme Outervision power estimator This is the most used and best of these tools, but it uses wattage not 12V amps.
Once you figure out how much power you NEED, you still should determine how much you WANT. Opinions vary on this, and it's a good way to start an argument on the boards... but I will say this: Buy a quality PSU that will last you for many years and cover any eventuality you can reasonably predict.

Glossary
Here is a glossary of terms that may be used in connection with PSUs, here and elsewhere:
http://www.tomshardware.com/forum/ [...] pply-terms

Now that you are armed with some basic knowledge, you might want to make up your own mind about which one to buy. Most websites, and indeed most so-called PSU reviewers lack the knowledge and the equipment to test power supplies correctly though. So where do you turn for expert reviews regarding power supplies?
Here are the top 3 websites that review PSUs
http://www.jonnyguru.com/
http://www.hardwaresecrets.com/
http://www.hardocp.com/

And here is a database of most power supply reviews from ALL the qualified PSU reviewers
http://www.realhardtechx.com/index [...] age541.htm

A List of Recommended PSUs
Were you hoping we would just tell you which PSU to buy? No problem. We have that covered too, with a list of the most recommended PSUs. Please note that this is just a partial list based on frequency of recommendation and such. There are many, many more good PSUs.
http://www.tomshardware.com/forum/ [...] s#t2430472

If you have general questions about this guide, or suggestions for it, feel free to make them here. If you want specific help about your build or intended PSU, ask in a new thread please.
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What are "12V amps"?

A PSU can deliver different kinds of power, but MOST of the power that a modern computer needs is the 12 volt DC kind.

So when you are looking for a power supply for a modern computer, your main interest is how much 12V power it can deliver. The 12V power comes on either a single rail (channel if you like) or multiple rails. Some major PSUs have just one rail and some have many... there is no clear right and wrong there, despite what the various companies may claim.


Let's look at a fake PSU label, so that we can see what it all means:




You'll notice the part we are really interested in is circled in red.

This particular PSU has two rails, apparently. They could be lying but we'll believe them. 12V1 and 12V2.

12V1, according to the chart, can deliver up to 30 amps. If more than that is drawn from this rail, it's supposed to shut down (If it's a good PSU).

12V2, according to the chart, can deliver up to 28 amps. Any more and it's supposed to shut down.

12V1 and 12V2 CANNOT deliver 58 amps combined. Very important. This is ONLY a statement of the maximum each rail can hit BY ITSELF.

We know that amps x voltage = wattage. 58A x 12V = 696W... but this is only a 640W PSU.

Jimbo has told us what these two rails are actually capable of though, and it's 520W.
All we have to do is apply the formula we just learned in reverse:

520W / 12V = 43.33 so we'll just say 43 amps of 12V power

There you have it, that is how we know where a PSU fits with others. We ignore the claimed total wattage and focus on how many 12V amps a PSU can actually deliver to you computer... because that's what matters most. MOST PSUs that are recommended have 12V amps that are reasonable for the total claimed wattage, but if you are trying to figure out if a particular PSU can power something, this should help.

I had thought about doing a post like this but i was afraid it would get too technical very quickly, hopefully you will be able to keep it all in layman's terms. Whats the next stage for this guide?

Hey proximon, can you elaborate on the 12v rails and the amperages? I am sure many people would like to learn about that stuff before buying their psu.

Nice job Proximon

The problem, as always, will be getting people to read the stickies.

I don't see any mention of checking for cable length before making the purchase.

For those that have a large tower chassis, this is very important. You don't want to be cursing yourself for not having taken this into consideration.

My page on PC power supplies from a gaming perspective has earned a lot of respect.

Most modular units use mini-fit junior connectors but some units like kingwins use different styles, kingwin uses little crystal cubes that are 9 pin connectors so for most of them the end that goes into the PSU is standard but there are issues with swapping cables between units. One issue with modular cables, especially units that are done up with entirely black wire to look nicer, is that the pin outs on the end of the connector that goes into the PSU can vary between manufacturers or even within a single manufacturer, so you need to make sure that if you are adding a cable from something other than the original packaging that it has the same pin out on the side that goes into the unit as expected, otherwise you can cause a short and damage something.

Extenders are always nice, perhaps there should be a discussion about adaquate cable lengths and how it relates to the size tower a unit is suitable for as some units don't have enough length to make a good fit for a case, for example the 8 pin connector on my EA500 is just barely long enough to make it to the 8 pin port on my motherboard so its very tight across my 4850, something that would have been good to know in the beginning.

You have it run over the card ?, not behind it ?
tecmo34's build, from his Step-by-Step guide in the New Build section
http://i964.photobucket.com/albums [...] C_0580.jpg

Nice upgrade, now you need to get some cards to utilize it

What are the 5v and the 3.3v rails used for then?

RAM lives off the CPU regulator, DDR3 needs something like 1.5 V if I remember off the top of my head. Less with the next step down in feature size.

http://www.hardcore-games.tk/wp/ram.php

My opinion;
For most people it's a waste of money.
Fact;
They are singular in purpose, test the psu prior to installation to see if it has the proper voltage's, done. ( Most people aren't going to be testing a pile of power supplies )

A digital multimeter can do the same thing and more.

The most recommended PSUs are not all of the good PSUs, as I have said. You may have access to PSUs made by FSP or Greatwall, for instance, that don't get recommended here but might be better options where you are.