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Everything you need to know about a power supply

Updated on January 3, 2018


This Hub is intended to explain what a power supply unit (PSU) is and what its specifications are. When you build or upgrade your PC the power supply isn’t that of an exciting piece of hardware to consider with. Sometimes people think of it at the last second or even as an afterthought.

I am going to tell you about what a power supply is and its purpose and also the specifications of a PSU.

What is a PSU

In fact this is one of the most important hardware piece in a PC, because every hardware component relies on it. It provides your PC and all of it’s components with the power it needs. The motherboard, the memory, all your harddrives, CD/DVD burners, fans etc. The bigger a PSU doesn’t mean your PC will run any faster but you will have more power to feed more additional cards, better videocards and processors.

So when you invest in a good quality well built PSU that easily handles all of the power consumption of your PC then it saves you a lot of headaches. A dead PSU is obvious and your PC won’t power up, but unfortunately some PSU issues are not that obvious. If you have a PSU that is failing or it can’t supply sufficient power to all hardware components it can boot up fine. But random crashes or a simple shutdown during gaming or working could occur due to this. So as you can see the PSU is very critical for the normal operation of your system, and therefore enthusiasts will ignore any cheap PSU. From personal experience I can tell you not to use any cheap PSU although it might be cheap it will affect your PC and its stability thoroughly.

So if you want a good quality high powered PSU you have to figure out which one will fit your PC needs and power requirements. If you only look at the PSU boxes on sites or in stores you see the wattage rating which is very important. But wattage isn’t the only thing and there’s more then meets the eye when choosing the right power supply.

This is the reason I’ve made this Hub and when you read it you know what PSU to buy and if it’s any good. Supply design, power delivery, the connectors and the efficiency will be part of your well informed purchase.


When you choice your PSU you have to decide which “form factor” you need so it will fit inside of your PC case. Pretty much all current desktops and tower PC’s will comply with the ATX (Advanced Technology eXtended) and PSU’s that fit into them are also branded ATX. So if you’re constructing a PC it is very easy to find out a case form factor because the specifications will be listed on the website where you’re going to buy it.

ATX specifies the positions of the screws on the back plate of a PSU, this is the part where you mount it into your PC case. On the right side you see a picture of an ATX PSU and the screw mounting points.

This standard defines a PSU height and width, but the depth can vary. If you have a high powered PSU like my 1000 watt PSU its depth will be bigger then the normal power supplies. So if you have a case that doesn’t leave a lot of room between your PSU and the back of an optical drive, you should avoid using a power supply that is deeper then the standard. Because the cables of the optical drive can interfere with those of the PSU. A normal PSU is about 5.5 inches in depth and a higher wattage and higher performance grade PSU as 1 inch extra.

On the right side you see a picture with a normal PSU build into a compact mini tower and a longer high wattage PSU placed on of it for comparison.

Also top mounted ports or fans in a PC case can get into the way so look out for potential problems inside of your case up front. Also all the PSU have at least 1 or even 2 cooling fans built in and those fans range from 60 to 140 mm. The larger a fan the less noise so the better it will be and therefore a 120 or 140 mm is desirable because larger fans move more air per revolution, and they are designed to spin slower which means less noise. But 140 mm fans are to big for the back so they are mounted on top, so be sure you have enough clearance around the fan so air can be pulled freely. On the right side you see a picture of a 140 mm fan.

Some ATX PSU’s also have fan speed that are adjustable that are based on power demand or heat. If you want a low noise PSU you could consider this feature to.


Some time back we saw a brief push for a new standard form factor for cases, PSU’s and motherboards which is called BTX (Balanced Technology Extended). BTX offered a better cooling solution but due to the shift to cooler processor design it eliminated the need for the BTX form factor and it faded into obscurity.

So if you have a PC from the mid 2000’s you should check if you have a ATX or BTX standard before you purchase a new power supply, because BTX cases won’t take any ATX PSU just like that.

On the right side you see a picture of a BTX case and a BTX power supply on the bottom right.

Compact and proprietary PSU’s

PC’s that aren’t a full or medium sized tower often have different power supply universe. Small form factor PC’s like Media Center or slim line stylish micro towers use mATX (MicroATX) or Mini-ITX cases and they require a rated PSU that matches. The dimension of these PSU’s is even smaller because the space tolerance is even tighter in the cases. So because they are much smaller and therefore also much lower wattage output than normal ATX PSU’s they lack the connectors for peripherals that are advanced like high end GPU’s. You can find everything you need to know about GPU's in my other Hub called: Everything you need to know about a videocard.

Some extreme compact PC’s from the manufacture Shuttle use proprietary PSU’s designs. So if you want or have to replace these PSU’s often you need to contact the manufacturer to do it.

On the ride you see Shuttle Barebone that is very compact.


Antec has introduced an intriguing form factor into a mix with a new CPX design. CPX PSU’s are bigger than the ATX counterparts, and they have 120 mm fans that are mounted on the rear and they currently fit in a few Antec cases only. Like the P183, P193 and Twelve Hundred are cases due to there size will fit those CPX power supplies. Until now no other manufacturer of power supplies has embraced this standard.

On the right side you see a CTX power supply.

Wattage and Rails

The next step you have to consider is making a choice for a PSU with enough wattage to provide the adequate power all your components require. If you look at all your individual components you get a general idea of how much power you need. For instance if you have a GPU that is high end edition that would require at least a 400 watt PSU but you also have a high end CPU (like an Intel I7) and multiple harddrives. If you want to find out about harddrives you should read my other Hub called: Everything you need to know about a Harddrive. So that means you require a higher wattage rating but assessing the requirements just from the head which isn’t that exact.

You can visit individual websites of the vendors of your hardware components and look at the power requirements of each individual piece. Such as the processor, harddrives, and videocard(s) together can make a total wattage requirement for your PC. But there is an other way to calculate your total wattage output and that is by visiting a site that has a power supply calculator that gives a fairly accurate calculation of your systems power needs bases on your choices. You select your PC components from a host of menus or checkboxes and based on that you get a pretty accurate picture of how much you’re going to need and so what PSU to buy. A long-time standard is the Outer Vision eXtreme’s eXtreme Power Supply Calculator Lite which is a very comprehensive free tool to calculate your total wattage. Also brands like Corsair,etc offer a recommendation based on there product for various configurations to.

Rails and Amperage

In addition to wattage there are other important terms like “rails” and “amperage” ratings. A rail is simple put an internal power path within a PSU that provides a single voltage. A PSU has typically a 3.3 volt rail, a 5 volt rail and one or multiple 12 volt rails. 12 volt is the rail which is critical these days because it’s used by the most demanding hardware in your PC, namely the CPU and the GPU.

Each device that plugs into a 12 volt connector requires a certain amount of amperage (amps). A PSU with a single rail has a single amperage rating but a multi rail PSU will break down this amperage on each rail. For instance Corsair singe rail HX750W has a rating to handle 62 amps maximum on its 12V+ rail. If we compare it to the Antec TP-850 which has 4 x 12V+ rails rated with 25 amps each.

The main difference is multiple rails supplies can’t necessarily supply all the amps which the rails are rated for at one time. To get back to the Antec TP-850 the total amperage available is not the actual sum of the individual rails because this would be a whopping 100 amps. But instead the PSU “combined amperage” spec is 62 amps. This combined amperage is given out to different rails if needed. So in the case of the Antec TP-850 that could be a 25 amps on each rail, but the total available amps the PSU can deliver is 62 and not the 100 amps of 4 x 25 amps rail. So if the 2 rails pulled a 25 amps each then 12 amps would remain for the other 2 rails to split up.

So are multiple rails better than 1 high amperage rail? Not always because the ATX specification limits each individual 12 volt rail to 20 amps out of safety purpose. But when higher amps turned out not to be a concern the limitation was dropped from the ATX specifications. The amount of rails matter more when individual rails carry less power, because you can max out the amperage that is available on 1 single rail system when you connect to many components that are demanding. These days PSU have over the 60 amps available on a single rail so this is much less of a concern. On the right side you see a YouTube clip of 3DGameMan and he explains about a single rail and multiply rails.

The totally available amps for 12 volt devices either they are on 1 rail or split between more than 1 rail doesn’t really matter. On the package you can find the specification sheet with a chart that breaks down the amperage on each rail available. Also this information you can find on the web sites of manufacturer’s.

The GPU is the biggest reason you have to pay attention to the amperage demands. Because it’s the most demanding hardware component (amperage wise), and there you should check the specifications on the website for the minimal requirements. So if you have one of the latest videocards the recommendation will be you should at least have a 600 watt PSU.

PSU Cables

When you choose a PSU you also have to look at the right connectors for you devices in your PC. Some newer PSU’s don’t accommodate any older components, so therefore make an inventory of the power connectors on all your components (motherboard, harddrives, expansion cards) that require a dedicated power connector. So when you choose your power supply keep in mind what devices are power hungry and ensure your new PSU has the power leads you need for it. Also you can find adapters that convert like a 4 pin Molex connector into SATA or floppy connectors.

The first thing you have to settle is the cable design. Some PSU’s come with all of there cables in place fixed while others have modular cables which allows you to click in only those cables you need. And some have a mixture of both hardwired cables and modular cables.

Modular cabling has the advantage of that it doesn’t clutter your PC and you have to find places for unused cables. This means less clutter in your case and a better airflow without any addition cables hanging down. Modular PSU’s have one little disadvantage they tend to cost more and you need to store the cables you don’t use carefully, in the event you want to add any new devices to your pc later on.

Motherboard and CPU Connectors

The first connectors you need to consider are your motherboard power feeds. Generally 2 connectors are required for all the current ATX motherboards namely the primary connector which carries all the voltages (20 or 24 pins) and a secondary 12 volt CPU connector that provides power to the processor (4 or 8 pins). Recommended is a PSU that has support for all these possibilities. It’s best to have a 24 pin main connector that has 4 pins that can snap off to create a 20 pin connector. Also an 8 pin CPU connector that can be snapped apart to create 4 pin connector. These power connectors are referred to by PSU sellers as “20+4” for the 24 pin main connector and “4+4” for the 8 pin CPU connector. On the right side you see what a 20+4 connector looks like.

The same principle goes for the 4+4 CPU connector. The availability of a 20+4 and a 4+4 connector is vital so you can use it for any ATX motherboard. Powersupplies are also sometimes labelled with “ATX12V” or “EPS12V” These terms signify of the existence of a CPU power connection on a PSU. All the new powersupplies you buy these days have it. Usually an ATX12V term indicates a 4 pin processor connection while the EPS12V term has an 8 pin connector. But this labelling isn’t that consistent because many recent PSU's that are listed as an ATX12V label have in fact an 8 pin CPU connector. Some have both a 4 pin and an 8 pin connector on the same cable like you see on the right side picture.

PCI Express Connector

Mid-range and high end videocards need 1 or 2 dedicated power connectors directly into the GPU, because up to date high end GPU’s draw so much power the PCI Express expansion slot can’t provide enough directly. So these GPU’s specific connectors are named after the expansion slot namely PCIe power connectors. To read about PCI Express slots you should read my other Hub called: Everything you need to know about a videocard.

PCIE Connectors come in 8 and 6 pin varieties and different GPU’s demand 1 or 2 of either of them or both. On the right side you see the 6 and the 8 pin connector.

Like the motherboard connectors these PCIE connectors have 8 pin connectors that can be snapped of to 6 pins (6+2 pin connector) to fit any configuration. Also make sure your PSU has enough connectors for videocards you’re going to install because some very hungry GPU’s require 2 PCIe connectors. 6 pin and a 8 pin or a 6 pin and a 6 pin or a 8 pin plus a 8 pin.

Also some of the PSU’s are compatible and labelled for Crossfire and/or SLI Ready. Read more about Crossfire and SLI ready in my other Hub called: Everything you need to know about a videocard. And this means the PSU has passed compatibility testing with multiple ATI or Nvidia videocards.

4 pin Molex

Most of the connectors on your PSU are Serial ATA and Molex connectors. Read about Serial ATA in my Hub called: Everything you need to know about a Harddrive. Make sure your PSU has enough connectors (type and the number) for your harddisks, optical drivers and other devices. Older ATA/IDE drivers use a 4 pin power plug and they are called Molex connectors. It has a sort of roughly “D” shape profile when you look at the end on. On the right you see such a power connector.

Serial ATA

The newer drivers use a Serial ATA interface and these are thin flat connectors called SATA power connectors. The have a sort of L shape groove in the leading edge. On the right you see such a connector. The recent PSU’s have enough of those connectors for a nice distribution of drivers, but in case you have an unusual amount of them in your PC or a bunch of other accessories that uses Molex connectors, you better count and check it.

Floppy Power Connector

Although it takes you back to older times, the 4 pin floppy drive connector is still relevant. On the right you see a picture of it. These days some of the PSU’s don’t have them anymore because of the fewer use of them in newer PC’s. But certain peripherals like card readers, soundcards or fan controllers use this connector as power.

Power Efficiency

The power consumption isn’t proportional directly to a wattage rating. Some PSU’s that depends on their design have more efficiency at passing power to hardware without wastage compared to others. The measurement of this is known as the “efficiency rating” of a supply and is expressed in a percentage. You should consider this number whether you want to be green or to save some money with your electric bill. The efficiency rating gives an indication of how much energy a PSU can deliver in wattage divided by the amount of energy that is fed into it. For instance you have a 600 watt PSU with 70 percent efficiency that would mean it supplies 420 watts of power to your PC, with a wastage of 180 watts given off as heat. So the higher the efficiency percentage the less wastage your powersupply will have.

Not only does it waste less energy but it will run cooler so it tributes to keeping the system heat and fan noises down. Apart from the heat and electric saving costs the higher efficiency rating is very desirable for computers with an interior that is crowded like the small form factor models.

The easiest way to discover how efficient a PSU is is by looking for the “80 Plus” Logo on it. On the right side you an 80 Plus Silver logo. This 80 Plus program mandates that a PSU is 80 percent efficient or higher at 20 percent, 50 percent and also 100 percent of the rated load, combined with a “power factor” of 0.9. The power factor stand for an advanced concept which is part of the 80 Plus specifications. About the power factor I’ll leave this out of this Hub because in my opinion this Hub will be getting to long.

Later on this program was expanded with 5 additional specific ratings namely the 80 Plus Bronze, 80 Plus Silver, 80 Plus Gold, 80 Plus Platinum and the 80 Plus Titanium which indicate an even greater efficient power management. On the right side you see a picture of all the 80 Plus ratings with there efficiency at 20%, 50% and 100%.

The chart on the right outlines the minimum required efficiency by the PSU 80 Plus compliant program at a certain load level. An important note is that the higher 80 Plus rating (Gold and Platinum) has to have an even higher efficiency at half the load. And a power supply spends most of its life working at this load rather than at maximum or low load.

These days most of the mid-range and high end PSU's have a level of 80 Plus certification. It’s one of the most important features you should look for. You see significant improvements if you go for a PSU with some level of 80 Plus logo when you compare it to a cheaper which hasn’t got one. If you buy a cheap PSU this will eventually cost you some money over time, because of the higher energy cost. The question what kind of 80 Plus certification to go for I would personally say go for the highest one (Titanium). It will give you the best power management the highest efficiency and the highest quality components inside of a PSU. But if you don’t want to spend so much money a bronze or silver is good enough!


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    • profile image


      5 years ago

      I took out the power supply from a Pentium 4 into a Pentium 3. It actually did work but when I connected the cpu to the monitor via the vga cable it doesn't display anything.

    • profile image


      5 years ago

      I took out the power supply from a Pentium 4 into a Pentium 3. It actually did work but when cpu is connected to the monitor via the vga cable it doesn't display anything.

    • profile image

      Tanweer Alam 

      5 years ago

      How to work lenovo's (G580 ) smart fan noise control

    • fkagchel profile imageAUTHOR


      6 years ago

      What sort of problems?

    • vinner profile image


      6 years ago from India

      My power supply unit is creating problem continuously. Is it because of faulty earthing?


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