Thursday, September 19, 2013

Computer Power Supply Buying Guide

Good Morning Everyone. Anyone interested in Computer Stuff knows about power supply. Still we often get confused about which power supply to buy for our system. So, I'm going to write a post about computer power supplies. 

What is a Power Supply?

We know desktop computers run from AC 110V/220V. But the internal components do not run on alternating current, neither run on such high voltage. Different computer component runs on different voltage level. For example, a 3.5inch hard drive requires both 5V and 12V DC supply. Now, How can I provide that voltage? Simple using an adapter. PC power supply is that adapter. It sits inside of your system and convert its input 110V/220V AC to different DC levels. Standard power supply has 3.3V, 5V, 12V, -12V, 5VStandby output leads. Sometimes they have color coding. 

This is a 850W power supply from corsair. You can see its large cooling fan and bundle of wires. Those wires power up different devices inside of a computer..

Though power supply does not directly improve your system's performance it still is a vital element of the computer. You will be needing a proper power supply for systems durability and reliability. If you don't have the right power supply unit, your system may turn off without notice, power supply might gets burnt. So, a right power supply is very important. 

Now few basics about the power supply.


All the power supplies you will see, you will find a Wattage rating printed on it. In most cases this is the amount of total power it can provide. Some power supplies print their continuous power and some print peak power. Remember always go for the higher continuous power rated power supply unit. So, if a power supply has a continuous 500W supply and one with a 500W peak, obviously go for the 500W continuous. This rating is actually the combined output power of all the different voltages.

This is the rating of a Thermaltake Toughpower 1000W. Its printed on the side of the power supply unit. As you can see the 1000W is the combination of all the different voltages. 


When looking for power supplies you may have come across the word Rail, Single rail, dual rail, quad rail. It means the output leads. Like the 5V rail which provides 5V, 12V rail which provides 12V. But if it has Dual rail or Quad rail it means there are Dual and Quad 12V output port. In the above picture you can see there are multiple +12V. It means it has multi-rail.

For safety manufacturer divides the total 12V Output current in different 12V rail, each having a maximum amount of current limit. For example if a power supply has dual 12V rail each of them has 20A rating, that means this power supply can provide (20A+20A)*12V=480W in its 12V rail(Make sure to look for the combined total output power of 12V rails,some power supplies have low total output current, if a power supply has 20A rating on both of its 12V Rail but a 23A combined current rating,that means you can't pull more than 23A or 276W from its 12V rail). Anyway, right, 480W on its dual 12V rail, but what will happen if you hook up 300W on it, and somehow you hook up all the load on a single 12V rail. It won't run because it has a 20A current limit on its each 12V rail. That means each of the rails will provide 240W. If you want to use more than that you have to distribute the load on different rails. 

That's the reason I prefer single rail power supplies, it provides all the current on a single rail, so you don't have to worry about connecting the loads in right places. If you have a multi-rail power supply just don't hook up all the load on a single wire, use multiple wires to balance the load.

Most important Rail

+12V supply rail is the most important rail of a power supply unit. If you use a gaming system or heavy system look for the power supply with highest +12V current rating. If you plan on buying a lets say 550W power supply, you find two of them from different manufacturer one with 42A on its 12V rail, other has 44A on its 12V rail, buy the one with the 44A. Because all the heavy things on your computer depends on the 12V rail. Your computers motherboard will take in 12V supply via a 8pin or 4pin ATX connector and convert it to the appropriate voltage for Processor, RAM etc. If you use a mid to high end Graphics Card it will take in 12V via 6pin or 8pin PCI express power connector and convert it to proper voltage for GPU core and memory. Your optical drive or hard drive's motor spins using the 12V supply, cooling fans run on 12V supply. So, 12V is the most important thing, just buy a power supply which has a high +12V current.


Power supplies have different types of connectors for different device. 

In the picture above you can see the 20+4 pin main power connector, 4 pin and 8 pin processor power connector and 6pin PCI express power connector. If you look at the motherboard you will see there are two atleast two different points for plugging in the power supply unit cable. One of them is the 20/24 pin power connector, other is the 4/8pin CPU power connector. If you buy a motherboard that has 8pin CPU power connector make sure to get a Power Supply that has a 8 pin power connector. There are more connectors for Graphics card, Hard drive, Optical drive, Cooling fan etc. Graphics cards usually use 6pin/8pin power connectors, Make sure to buy a PSU with proper connectors for your system. Hard drives and Optical drives this days require SATA power connector(4/5pin) and cooling fans are often powered by Molex(4pin) connector.

One of the important thing about power supply is the MTBF. It means Mean Time Between Failure. This is the approximate time that the Power Supply Will serve without any failure. Higher MTBF value means higher service hours. So, better taking a Power supply with high MTBF like 100000 Hours or more. Don't buy any power supply below 20 000 hours of MTBF. 

Remember power supply will only reach its MTBF if it runs at proper condition. You can't overload it, you can't run it in high temperature.


Efficiency is also an important thing. Efficiency does not affect its output power but it affects its input power. Lets assume two power supplies with same power rating of 500W providing constant 200W to the load, one with a 60% efficiency and the other with a 85% efficiency. 

Then the one with 60% efficiency will  pull around 333W From AC input.
And the one with 85% efficiency will pull around 235W from AC input.

So,higher efficiency will surely cut your electric bill. For a greener world use power supply with high efficiency.


Power Supply has transformer, DC to DC converter inside of it, which often get hot. To cool it down power supplies come with heatsink and fan inside of it. Some power supply has 8cm fan some has 12cm or even 14cm. Get a power supply which has a good cooling solution. The better the cooling fan is the better the heat dissipation will be. And it will result in longer power supply life.

Power Supplies with different sized fans.

Non-Modular/Semi-Modular/Fully Modular

You may have also heard these terms. It is the cable management feature. If you buy a normal non-modular power supply, All the cables will be attached with it, you can't remove them. So huge amount of cables will be lying inside of your system which can cause poor airflow inside of your case. Power supply cables blocks the airflow, more cables more blocking. Most of the time you won't be needing all the connectors but with the non-modular design you still will be needing to get them connected.

With the semi modular design only a few cables like the 4/8Pin power connector and 20/24pin connector is attached to it. It gives you the flexibility to use cables as you want. Use the cables that you need. Don't need to connect the other cables so, eventually it will be less jampack your system and will result in better airflow. This feature will also allow you to route cables as you want.

Fully modular has no cables connected (like the one in the above picture). You have to connect all the cables.

My choice is the semi-modular because we will need those cables no matter what. And each contact point may have electrical resistance, which  may lower the power. If you think about the resistance that might be present in connectors, Non modular will be best choice, but we need cable management for good airflow, so if we think about balancing it, Semi - Modular will be the best choice.

Support And Protection

If you plan on building a very high end system with multiple Graphics Card, you will need a certified power supply. The most common multi-GPU system is SLI or Crossfire make sure the power supply has these certificates. 

Good power supplies feature over current, over voltage, under voltage protections. These are very important feature. Accidental short circuit may blow up the power supply if it doesn't have over current protection feature. Moreover if accidentally load gets higher than its capability it will turn off automatically.


PFC stands for power factor correction. As this device will be running from AC, power factor is a very important thing here. In DC , watt is the product of voltage and current. But in AC watt is the product of voltage, current and power factor. So the better the power factor is better the power supply is. Modern good power supplies come with power factor correction mechanism. This mechanism automatically adjust the power factor to higher value. It results in lower power consumption. Efficiency and Power Factor is not the same, though they have almost the same effect.

So, that is all about power supply in brief.

Now lets take a look at which one to buy.

 1. Calculate your consumption. Find out how much power you will need. Get a power supply with atleast 20% higher wattage than your consumption. If you plan on overclocking buy even higher.

Remember your power supply will have the best efficiency when it will run on 50-60% load. So, if you have enough money you can buy such a power supply which has 200% rating of your consumption. It may require more money but eventually will pay off. More over lower load means lower temperature of the Power Supply Unit and lower noise from its cooling fan.

 2. My suggestion will be go for the single rail. 

 3. Study the power ratings carefully and buy the power supply with higher +12V current.

 4. Make sure the power supply has enough connectors for all your devices.

 5. Buy the one with Higher MTBF value.

 6. Buy the one with Higher Efficiency(must buy 80+) and better cooling option.

 7. My suggestion, semi-modular.

 8. Make sure to check for features like low voltage , over voltage and low current protection.

 9. Make sure it has Active PFC(Power Factor Correction)

10. Must have better warranty period.

11. Don't buy cheap or non-brand power supply. 

List of Good Power Supply Manufacturer

 1. Antec
 2. Silverstone
 3. Corsair
 4. Seasonic
 5. OCZ
 6. Rosewill
 7. Thermaltake
 8. Cooler Master
 9. Sapphire 
10. XFX

Hope it will help. That was all for now.

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