How to choose a good Power Supply Unit

[zoheb]

What is a Power Supply Unit?

The power supply unit (abbreviated as PSU) or the SMPS, as it is more commonly known, is a device that supplies power to all the components that make up a computer system.

Why should I give a damn about it?

This is a very pertinent question that many computer buyers ask. Up until 2006, choosing a good power supply was not a major issue. The PSUs that came coupled with our cabinets were quite sufficient to power the various devices in our computer. One can say that choosing a good PSU became a major concern only with the advent of high performance PCI-Express based graphics cards. These graphics cards would require a separate power supply connection obtained directly from the PSU to satisfy their power requirement.

Suddenly the least thought about component in a computer system became a point of major concern. What is the efficiency of the power supply? What wattage does my system require? What amperage is required on the 12V rail? You could almost hear the collective gasp of a million first-time computer buyers from a continent away (and this sound continues to echo even today).

Ok fine, so your precious PSU is important. What do I need to see when I choose a good power supply?

Now that's more like it! You should look for the following things when choosing a power supply for your system:

1) Rated power output in Watts (quite obvious, don't ya think?)

2) Total amperage on the various rails (more on this later)

3) Connectors included with it (6-pin PCI-E connector, 2x6-pin, 4x6-pin, 8-pin)

4) If running SLI/Crossfire, whether the unit is SLI/Crossfire certified

5) Modularity (or the lack of it)

6) Efficiency of the PSU

We'll deal with these requirements one by one.

Before we proceed further, it is important to know the difference between PEAK rated output power and CONTINUOUS rated output power.

CONTINUOUS rated output power is the power output that the PSU can supply continuously under normal operation. PEAK output power is, well, the maximum output power that the PSU can provide. Obviously, continuous output power is of more interest to us.

Some PSU manufacturers will mislead consumers by specifying the peak output power on their PSUs, while having a continuous power rating much below the specified value. An example of this would be the Coolermaster Extreme Power 430W. This unit supplies 430W of peak output power, while the continuous output power rating of the PSU is 400W. Differentiating between continuous and peak output power is important while choosing a suitable power supply.

What do these 12V, 5V etc. rails mean?

Now, a PSU will obviously provide a voltage (and current) output, and this voltage output is divided into different levels, or "rails". This division is done because different devices in the computer require different voltages. For eg. the 3.3V level is required for the CPU and the memory, the 12V level is required for the hard drives, graphics cards etc. Each rail will also have a current output rating, or "amperage". This amperage is important to know if the power supply is capable of providing the minimum current required for the various devices.

All that is fine, but what do multiple +12V rails mean?

PSU manufacturers will often times split up the 12V output voltage provided by the power supply into multiple rails. The only reason for doing this is "safety". For eg., consider the following 2 cases:

1) A PSU with 54A on a SINGLE +12V rail

2) A PSU with three +12V rails, with an amperage of 18A on each rail

Which of the above would be safer? Logic would say PSU no. 2.

The goal here is to limit the current through each wire to what that wire can carry without becoming dangerously hot/catching on fire. Most PSUs these days come with multiple +12V rails, so that the required current can be supplied to each device safely. But as we shall see below, this is really not a very important specification.

Does it really matter, whether a PSU has a single +12V rail or multiple rails?

The answer is NO. As long as the PSU is able to provide the required amperage on the +12V rail that the devices using it require and is made by a reputed brand, there is no advantage (or disadvantage) to splitting a +12V rail into multiple ones. This is a false presumption, and one that even the PSU manufacturers are responsible for spreading.

Is it true that some PSU's that claim to be multiple +12V rails don't have the +12V rail split at all?

Yes, this is true. It's typically seen in Seasonic built units (like the Corsair HX and Antec True Power Trio.) It's actually cheaper to make a single +12V rail PSU because you forego all of the components used in splitting up and limiting each rail and this may be one reason some OEM's will not split the rails, but say they are split. Some system builders adhere very closely to ATX12V specification for liability reasons, so a company that wants to get that business but also save money and reduce R&D costs will often "fib" and say the PSU has it's +12V split when it does not.

Why don't those PSU companies get in trouble?

Because Intel actually lifted the split +12V rail requirement from spec, but they didn't actually "announce" it. They just changed the verbiage from "required" to "recommended" leaving system builders a bit confused as to what the specification really is.

What is a PCI-E power connector?

A PCI-E power connector is basically the wire from the PSU that will go directly to your PCI-E graphics card. It is important to know how many PCI-E connectors are required for your graphics card(s) and how many are supported by your PSU. For eg. if you are planning to buy a single Radeon 4850 gfx card, you will require a single 6-pin PCI-E power connector. A Radeon 4870 on the other hand will require TWO 6-pin power connectors. Two 4870s will require four 6-pin power connectors and so on.

My PSU has only 2 PCI-E power connectors, but I need 4 for my SLI setup. Help!

Fear not, in this case you may use the 2 PCI-E connectors for one of the graphics cards in your SLI setup, and use two Molex to PCI-E adapters for the second card. However, this approach has a problem: When you use the Molex to PCI-E adapters, you have now added the load from the graphics card onto the rail that's also supplying power to all of your hard drives, optical drives, fans, CCFL's, water pump.. you name it. Suddenly, during a game, the PC shuts down completely.

To avoid this, always choose a PSU with sufficient PCI-E connectors, for your present (as well as future) needs. Remember, the PSU will be used in your PC for a minimum of 5 years, even if you upgrade your PC once a year.

What are SLI/Crossfire Certified PSUs?

This certification really means nothing if you know what you want in a power supply, but for the newbie, this certification indicates that the power supply (most probably) meets all the requirements of an SLI/Crossfire gaming rig. However, going by this certification is not enough, it is important to see the specs of the PSU for yourself and ALWAYS buy from a reputed brand.

What are Modular PSUs?

A modular power supply is a relatively new approach to cabling, allowing users to omit unused cables. Whereas a conventional design has numerous cables permanently connected to the power supply, a modular power supply provides connectors at the power supply end, allowing unused cables to be detached from the power supply, producing less clutter, a neater appearance and less interference with airflow. It also makes it possible to supply a wider variety of cables, providing different lengths or Serial ATA instead of Molex connectors.

Modular PSUs are quite popular right now for the above advantages. Obviously modular PSUs cost more than equivalent conventional PSUs, so it is up to the consumer to decide if the benefits of modularity justify the extra cost.

What is efficiency?

What you should know is that the PSU basically converts the AC power (supplied by your state's electricity board) into DC power that is required by the computer's components.

Obviously, this conversion is not 100% efficient, with some amount of power being dissipated in the form of heat. Efficiency of a PSU can be formally defined as the percentage ratio of the DC ouput power supplied by the PSU to the AC input power taken by it. Obviously, a higher efficiency PSU will require LESS AC input power to give a specified DC output power, than a lower efficiency PSU.

Eg. Consider a 500W PSU rated at 80% efficiency and another 500W PSU rated at 70% efficiency. The first PSU will require 500/0.8 = 625W of AC input power, while the second one will require 500/0.7 = 715W of AC input power (considering both PSUs to be working at full load). Almost a 100W difference between the two. And where does this wasted power go? It gets dissipated in the form of heat. So lower efficiency PSUs dissipate more amount of heat than equivalent rating higher efficiency PSUs. Heat dissipation can only lead to shortened life span of your power supply, so it is important to choose a high efficiency PSU, not only to save electricity bills, but to ensure long life for your PSU.

Another thing to note is the room temperature for which the PSU was rated. As most people know, efficiency of such devices decrease with increase in temperature. Some PSU's like the Seasonic ones are religiously rated at 50 deg while most others are rated at 25deg. So the capabilities of a 500W PSU from two different manufacturers may actually be different. This also means that a manufacturer who sourced a 500W PSU tested at 50 deg from an OEM may put a label of 650W or so at 25deg.

Apart from these basic requirements, it is always good to buy from a reputed manufacturer. Corsair, Antec, OCZ, Silverstone, Tagan are some good power supply manufacturers. VIP (the really el-cheapo ones), Mercury (all of them), Coolermaster (the Extreme Power Series) are NOT (although Coolermaster is way better than the other two). A good brand for the budget-conscious buyer would be FSP. ALWAYS choose a good brand for your supply. I cannot stress this enough. It is probably the single most important component you will buy for your computer, don't cut corners here.

[Xilo]

Good information. NEVER skimp on the PSU. The PSU is literally the most important component in your PC. If it goes bad you're out of a working computer and it can fry other components. I've seen a PSU shoot fire out the back.

One brand to stay away from is Ultra. They make absolute ****ty PSUs.

[OxideNOS]

zoheb, very good and useful information to select the right power supply. PSU is one of the best investment that will retain its value for many years.

[+shift. MVC]

I do have to note that regarding split +12v rails, it really depends on HOW your rails are split and whether the way that rail is split is appropriate for the setup in question.

There ARE some situations albeit rare ones where because of the way the rails are split it is not actually the ideal situation.

I'll also add that efficiency awards given by 80 Plus are done at 23C.

Anyway, if you wrote this yourself very nice article, even if you copy and pasted from elsewhere - very nice article!

[ViZioN]

Nice article (Y) I learned a few things. Sticky!

[TokiToki]

Cite your source?

http://www.techenclave.com/pc-buying-advice/the-power-supply-unit-recommendation-thread-155724.html

[InterceptorX]

Cite your source?

http://www.techenclave.com/pc-buying-advice/the-power-supply-unit-recommendation-thread-155724.html

Thinking the same thing... Wish I could take that rep point back now...

[+LogicalApex MVC]

Cite your source?

http://www.techenclave.com/pc-buying-advice/the-power-supply-unit-recommendation-thread-155724.html

Whoa that's pretty nasty... Unless he can prove that he is techster this whole thread should be locked and deleted. Plagiarism is pretty nasty.

[random_n]

I've got both of Silverstone's fanless PSU models for my two desktops. They're both heavy as bricks like most of the IBM xSeries server PSUs I come across, and keep my systems stable as a rock. Motherboard and PSU are two components where cheaping out isn't a great plan.

Nice article, techster!

[Barney T. Administrators]

Needed to cite the source. We do not allow plagiarism.

Thread closed