Power Supply 101: Understanding Power Supplies and Selecting the Right One for the Job

Not open for further replies.


Sep 12, 2013
Don't forget to check out other 101 articles:
Airflow]http://www.tomshardware.com/faq/id-1858957/airflow-101-setting-fans-keeping-computer-cool.html]Airflow 101[/url]
Power Supply 101


Welcome to Power Supply 101! If you've come here because you're new to PC Building, or if you're a veteran who just wants to see if there are any ideas you can use, you're in the right place. I geared this as being a basic rundown, so if there are some parts missing, don't worry, this isn't meant to be an all-encompassing guide.

The power supply (PSU) is all too often viewed as one of the components that doesn’t need too much attention paid to it during its selection. Many people think that any power supply will do just as good as any other one, and that as long as it meets the wattage requirement, it will run perfectly fine. Unfortunately, this all too often leads to a myriad of problems and in some cases it can even lead to hardware failure.

Power Supply Quality is Important
This is to some a dramatic understatement, but this is such an important concept it needs to go at the beginning of this article. If you don’t want to read any more into the article, remember this section.

The most common reason people run into power supply problems is their budget. When beginning (and even some experienced) builders start to reach their budget’s limit, the power supply is all too often the first to meet the chopping block. This comes down to how people compare PCs. How often have you heard “My PSU is so much better than yours”? Chances are you haven’t, and that’s because everyone is talking about their CPUs and GPUs. A builder wants the parts that they can show off, even if it means cutting other components.

Don’t take this to mean you have to sink a large amount of money into a power supply, that isn’t the case. There are many PSU vendors that sell high quality power supplies for reasonable prices. What you should take away is that if your CPU is the brains, and your GPU is the brawn, the PSU is the heart, and without a good heart, you won’t get very far.

What Does A Power Supply Do?

A big pile of power.

Before we can look in depth at power supplies, we have to understand what they do. The good news is that we don’t need an electrical engineer’s knowledge, just a basic overview.

Power supplies, like the name implies, are the primary supplier of power to your motherboard. They do this by converting the high voltage alternating current (AC) that comes out of the wall socket (in the range of 110V to 240V depending what country you live in) into usable low voltage direct current (DC). This happens through a process that while interesting, is beyond the scope of what this tutorial in set to teach. The result is power that can now be used by your PC to power the components. This power is broken down into 3 voltages: 12V, 5V, and 3.3V. Different components in your computer will use different voltages; however, it is important to note that most modern computers receive roughly 80-90% of their power from the 12V and thus most modern power supplies send the majority of their power across the 12V rail(s).

Power Supply Form Factors

While there are a plethora of form factors for PSUs, the ATX form factorthe most common form factor, and the majority of the time, what you will see in personal computers. When selecting a power supply, unless your case requires a different form factor (this is most prevalent in very small cases and you will be informed) the ATX standard is the go to standard for PSUs.

Understanding Power Supply Units

When choosing a PSU, you will often hear people talking about Amps, Volts, and Watts. These are the three most common units you will interact with when you are looking for a power supply. What many people fail to recognize is that these units are interconnected; this leads to some of the common myths behind PSUs, which we will talk about later.

The most important thing to remember about the PSU’s units is that they all interconnect through this formula:

Watts = Voltage * Amperage

This formula will be helpful in the next section where we look at PSU labels.

Understanding PSU Labels

The PSU label is a sticker on the side of your power supply that tells you everything your power supply is capable of doing. We will take a look at two labels and walk through the process of deciphering them.

The AC Input is the voltage range the power supply is able to convert into DC. Some power supplies will have a small switch on the back that lets you switch from 110V to 240V. This must be placed on the proper setting before you power on your computer or the PSU will not work (and may be damaged). Some higher end models are “auto-selecting” and do not have a switch on the back. Do not take the lack of a switch to indicate auto-switching, some PSUs will not have a switch and will only work in certain regions. Make sure to verify this before you buy (most e-retailers will have the PSU label posted in the images on the PSU’s page).

The DC output shows you the voltages at which power is being output. You can see the 3.3V, 5V, and 12V rails, in addition to two others. The -12V rail is no longer used, but kept on as legacy support. The +5Vsb is a small rail in charge of powering on the computer and keeping the BIOS memory powered. These last two rails are not factored into our combined wattage.

The Max Load is the maximum amperage you can have on a rail without the PSU shutting off. This will change depending on what the power supply’s total output is. This is also an important area when it comes to power surges. A 1200W PSU with a 100A 12V rail could easily destroy a computer in the event of a major power surge. This is one benefit of a multi rail system which we will touch more on later.

The +12V rail is what you should pay the most attention to when selecting a power supply since roughly 80-90% of a modern computer’s power is supplied through this rail.

This is an example of a multi rail system. We see that instead of one large 12V rail that can deliver 45 amps, we have four smaller rails that can collectively deliver 41 amps. Your first thought may be: Since when does 18 x 4 = 41? It doesn’t, the amperages for each of those rails is the max load, the point at which the PSU shuts down to protect itself. If any single rail reached 18A, the PSU will shut down. This is why it is beneficial to have a multi rail PSU for high wattages, you have a lower trip point which can save your system from a power surge or short circuit (versus our earlier 1200W 12V 100A example).

In the event that the PSU label doesn’t show you the combined amperage (which is very important during GPU selection) remember that you can calculate it with the formula I showed you earlier. In this case we would have:

Watts = Voltage * Amperage (In the case of a multi rail PSU this is the combined max load)
492 = 12 * Amperage (Comb.)
492 / 12 = Amperage (Comb.)
41 = Amperage (Comb.) - That checks out!

Congratulations, this was the longest section, and you’re done! To wrap up this tutorial, lets touch on some of the other important points of PSU selection, and some of the myths behind it.

80 Plus Ratings and Their Importance

80 Plus Efficiency Requirements

Simply put, 80 Plus is a rating given to power supplies based on their efficiencies at certain loads. It isn’t a good indicator of the quality of a PSU, since manufacturers may sacrifice other important things in an effort to achieve the certification. In addition, even though the PSU is rated at a certain efficiency level, environmental factors such as heat can rapidly degrade your PSU’s efficiency. In conclusion, don’t base your purchase solely on this, use it as an indicator.

Modular PSUs

Modular PSUs are a great form of PSU that gives you the option of what you plug in. This helps you keep your case clear of excess cables that you are not currently using. If your budget will allow it, this is one of the best features on a power supply.

Common PSU Myths

There are plenty of myths that plague the PSU world, and most of these come from only examining part of the information that is offered. Let’s look at three of the most common:

Myth 1: Amperage is the most important part in judging a PSU’s quality.

We know that most of the computer’s power goes through the 12V rail, and so the amperage running through that is very relevant to a PSU’s capabilities. That said, just like 80 Plus ratings, the higher amperage isn’t the end all determiner of PSU quality.

Myth 2: You should run your computer at 50% load for the best efficiency.

It’s true, PSUs are most efficient in the vicinity of 40-60% (look at the 80 Plus chart above). That said, chances are you aren’t pushing your system at 100% load for most of the time (I’ve spent the past couple of hours writing this article, with my computer pretty much idling). Not only that, but your power consumption changes frequently, depending on what you are doing at any given time.
So think about this, if you were to take two PSUs, one a lower wattage than the other, but with the same efficiency curve, which would be more efficient? The answer is the lower wattage, because you’ll be closer to peak efficiency in general. Your results would be spread out more over the curve with the PSU with a larger wattage. The difference is very small however, and your efficiency for the most part doesn’t change. The price however, might. Buying a 400W PSU versus an 800W PSU can be a large difference when it comes to price. Look for a PSU that meets your total power needs with just a bit of headroom, and be realistic with your future proofing, each generation of upgrades does not immediately bring a large increase in power consumption.

Myth 3: Single Rail is better than Multi Rail (and vice versa)

There is no major difference between these designs. Neither will let your computer perform better or overclock higher. The only benefit to a multi rail setup is an extra layer of protection at higher wattages when a surge or short circuit occurs.

Well, you made it to the end! Hopefully you can walk away from this with a much more in depth understanding of Power Supplies and the ability to choose a great power supply that meets your needs. It is important to note that as you become more experienced, you may end up doing things differently, and that's perfectly fine. This tutorial is only meant as a basic guideline, and it may not work in all the situations.
If you have any questions or feel that I missed something, feel free to ask or comment so I can update this guide to be most beneficial to you, the readers. A good power supply is worth the price, and it will cost you less in the long run if you choose a high quality PSU that meets your needs.

Happy Building!


Sources: The majority of this article is synthesized from multiple articles written by Phaedrus2129. This article only touches on the concepts he more fully covers and I recommend you check out the articles if you are looking for a more in depth read on specific topics or interested in which brands offer the highest quality PSUs, his articles are well worth the read. You can find his work here: Phaedrus2129’s PSU Articles

His writing through extension of mine is protected through the creative commons license.

Not open for further replies.