Power Supply

Power Supply Units (PSUs) have gotten a lot of traction lately due to the ever expanding power needs of enthusiast rigs. We have a plethora of technologies and products that help bolster our prejudice and at the same time the power requirements up a few points. There may never be an end to this vicious cycle but what good we can do is that we can build a rig with sufficient power that satisfies our current and near future upgrade needs. I aim to write an account of recent PSU innovations and match power requirements of our rigs with the power supplied by today's PSUs.

Measuring Power demand

First and foremost, there is an online free power calculator service that determines your system power needs. They have not only listed all the current CPUs, GPUs, optical drives etc but they also keep their website updated with the new product launches and all that. However, they don't legally claim the accuracy of their power figures. [NB I'm sure you know the legal BS of US firms]. In my experience this calculator gives a fair power figure that you can reliably use to make a purchasing decision.

Power Efficiency theory

Efficiency in a power supply can be defined as the power loss in AC-to-DC conversion, expressed as a percentage of total AC input power. For example, a power supply that requires 100W AC input to produce 70W DC output has an efficiency of 70%. In this example, 30W is lost as heat within the PSU. A power supply that requires 100W AC input to produce 80W DC output has an efficiency of 80%. 20W is lost as heat. The 10W difference in these examples seems trivial. However, at higher output levels, differences in efficiency becomes quite significant in terms of how much energy is lost as heat.

Different power supplies draw power with different rates of efficiency, but they (generally) only draw what they need. If the components in your PC only require 300W, a 500W PSU and a 350W PSU will each deliver 300W. Nonetheless, the PSU with the lower efficiency will draw more AC from your house to deliver 300W DC. A 300W labelled PSU does not mean that it always uses 300W, that just means that it is rated at being able to produce up to 300W of power. You may also notice that a 300W labelled PSU may not be exactly pumping out 300watts, it can be a real 275W PSU rounded off to the nearest hundred. Some companies rate them at the Max, others nominal, and some do round up. You will find PSUs that will not reach the rating without causing problems, and you will find some that will exceed the rating. The key is to stick with quality manufacturers that stand behind their product.

PSU with higher efficiency usually generates less heat and needs less forced airflow to stay cool. This means lower fan noise and longer life for the components. It also means less heat leaking out from the PSU into the rest of the system, especially to the already hot CPU, which is usually located right next to the PSU, within a couple of inches. Lowering the heat radiating from the PSU can help to lower the airflow requirement for cooling the CPU as well, leading again, to less fan noise.

PSUs come with their labels that give you an idea of what to expect from it. Say for example the following PSU label (Fig. 1.1) shows that the max power output is 650W. It is therefore not the continuous but the peak output. It also tells that this PSU uses four +12V rails to give this power output.

Fig. 1.1 [click to enlarge]

Ambiguous PSU label

On the other hand, the following PSU label (Fig. 1.2) gives a more consumer friendly description. This PSU is going to give a continuous 650W output at room temperature. It is also a three +12V rails design.

Fig. 1.2 [click to enlarge]

More descriptive PSU label

As a rule of thumb, don’t ever buy a PSU that feels light weight or the design of the whole thing feels cheap. This is because such power supplies are inefficient as well as inappropriate to give a decent continuous power output for your needs.

PSU connector types

Next is the new type of PSU connectors namely, SATA connectors (Fig. 2.4) and PCI Express, aka PEG (Fig. 2.5) connectors. Gone are the days when only Molex (Fig. 2.3) connectors used to supply all the juice. With BD-ROMs coming with SATA connectors and HDD totally shifted to SATA interface these connectors make a lot of difference. The following figures show the shapes of the different connectors of a PSU

20-Pin Connector [click to enlarge]

24-Pin Connector [click to enlarge]

4-Pin Molex Connector [click to enlarge]

Serial ATA Connector [click to enlarge]

PCI-Express Connector [click to enlarge]

The uses of these PSU connectors are as follows:

Power Supply Connector
20 or 24 pin connector*
4 pin molex
CD-ROM, Standard Hard Drive, Floppy Drive, Fans, Lights, Most Accessories
Serial ATA Connector
Serial ATA Hard Drive**
PCI Express Connector
Most newer Video Cards

* Adapters are available for converting 20 pins to 24 pins, and visa-versa
** A Serial ATA Power Adapter can be used to convert a four pin molex cable to a Serial ATA Power Connector

What about SLI?

The recent focus on multi-GPU card system has also raised the power demands from PSUs. On the one hand, we now have rather generous SLI Quad configuration systems and on the other there are systems with two Radeon X2 cards working in tandem. For this type of configuration, it is recommended that you pick an SLI Certified or CrossFire Ready PSU. It features multiple PCI-E power connectors as well as continuous power output that satisfy your configuration requirements. As a side note, if you are to buy an inexpensive SLI rig then you may choose a 500W – 600W PSU with 80% minimum efficiency at maximum load. By inexpensive SLI rig I mean, two 9600GT in SLI or two Radeon 3850 in CrossFireX setup. However, if you are in the market of expensive PSU then make sure that you buy one with modular and sleeved cables (Fig. 3.2). Modular PSU allows you attach/detach cables from it depending on your needs while sleeved cables have a protective coating around them that helps reduce the clutter and prevents from short circuiting hazards.

Standard Power Supply [click to enlarge]

Modular & Sleeved Power Supply [click to enlarge]

There are other considerations of power demands for liquid cooling and as a result overclocking the CPU, RAM, GPU etc. If you intend to go down that path in the future then purchase a PSU with enough beef left at the end. The question of “how much?” depends on the level of overclocking you are to achieve and your specific liquid cooling kit power demand.

Noise & Cooling

Generally, it is observed that PSUs leave a lot of cable clutter inside your case – I’m talking about unsleeved cables. It is important for you to sideline these cables so as to retain a healthy level of air flow inside the computer case. The ideal case flow (Fig. 4.1) involves 4 fans of 80mm (average RPMs) with two fans for air intake and two for exhaust.

Fig 4.1 [click to enlarge]

Ideal Case Air Flow

PSUs also come with their share of noise and heat dissipation. With the ever increasing noise of computer components, it is important to build a rig with minimal noise coming from each component. A PSU with good heat sinks and/or low noise but efficient fan is a definite plus. A cool PSU gives a better continuous power output so efficiency is usually on the higher side.


Lastly, we have the brands to choose from. Asus, Corsair, OCZ, Cooler Master, Zalman, Gigabyte are all good brands to name a few. Just know your current and future needs, watch your budget and pick any 80-plus PSU that you like.


Changes made to 'Power Efficiency Theory' by FIREBLADE

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