Power supply PSU buying guide

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The power supply (unit - PSU) is a core component in any computer build. Choosing the right one is important primarily for cost, noise, life and reliability reasons. This guide is written to help decide on an appropriate power supply for your computer, and what features are available so that you know what to look for.

Modular supplies

This term refers to power supplies where you only plug in the cables that you need. This means that there are fewer cables in your case, which makes things tidier and easier to get access to, and also improves the airflow. Having better airflow is important as it means less dust stays there (more dust means more chance of components overheating and failing. Dust also keeps heat in, which means the fans run harder, which leads to a noisier machine), and that the fans don't have to work so hard to extract air whether there is dust or not, which also means a quieter machine. Having a modular power supply isn't essential, but does provide benefits.



Efficiency indicates how much usable power is provided by the unit for the computer, against what it consumes on your bill. Computer power supplies transform the power from the wall outlet (which is AC) into power for your computer's needs, which is DC. As part of that transformation, some energy is lost. If too much is lost and your components don't get the power they need, your computer will not work reliably. Less efficient units also mean a bigger power bill as they need to consume more power to deliver what your p.c. requires. The other downside with cheap units and poor efficiency is that the lost energy is given off as heat, requiring your fans to use more power and make more noise inside the machine to keep it cool.

80+% efficiency is good, 85% is excellent. Some generic supplies may only be 50% or less efficient.


Watts vs Amps

The efficiency of the unit mostly refers to watts, a 50% efficient 600w unit is only delivering 300w of usable power. However, the supply is provided along more than one source, or rail as they are called. The 12v rail is the most important one (some power supplies have more than one, which is better but not essential) as that rail is used for both the CPU and graphic card. The output on these rails is given in Amps.

Power supplies should be checked to see what the DC output on the 12v rail is. Some cheaper supplies have a high wattage but only deliver 20A on the 12v rail, which is insufficient for a typical, current machine.


How much power do you need?

It is best to use an online calculator for this, where you enter in details of your other components to assess how much power your machine needs. Unfortunately, external sites cannot be linked from ebay guides, but if you do an external search for an 'extreme power supply calculator' that should help you find one. As a rough estimate, as long as the rest of your computer doesn't have exceptional requirements (which you'd be likely to know if it did) and hasn't been overclocked, here are some estimated Amperage requirements for the system based on more recent graphic cards as the graphics card is one of the heaviest power consumers in a computer. Older graphic cards or systems with onboard graphics will be below these:

Triple SLI 8800GTX/8800 Ultra/9800GTX or Quad SLI 9800GX2 – 60A minimum on +12V rails.
Crossfire HD 2900XT/3870X2 or SLI 8800 Ultra – 50A minimum on combined +12V rails.
SLI 8800GTX/9800GTX based system – 45A minimum on combined +12V rails, 50A+ recommended
SLI 8800GTS based system – 38A minimum, 43A+ recommended
SLI 8800GT or Crossfire HD 38x0 based system – 35A minimum, 40A+ recommended
HD 2900XT/8800 Ultra/HD3870X2 based system – 30A minimum, 35A+ recommended
8800GTX/9800GTX based system – minimum 27A, 32A+ or higher recommended
8800GTS/8800GT/x1900xtx/x1950xtx based system – minimum 25A, 30A+ recommended
8800GT/HD3850/HD3870/9600GT/8800GS – minimum 25A, 30A+ recommended
7900GS/7900GT/7950GT/1950pro/x1950gt – minimum 20A, 24A+ recommended
7600GT/8600GT/8600GTS/2600pro/2600xt – 18A minimum, 21A+ recommended

In summary, don't buy too low a power supply for your computer as you run the risk of reliability problems as components don't get enough power to work properly. Equally, there's no need to buy an excessively high power supply for the computer as it will cost you a lot more than you need to and will often result in higher power bills, and sometimes heat and noise.



Power supplies include a fan to keep them operating safely, without overheating. Designs and sizes vary with some techniques to try to minimise the fan noise by controlling the speed or using double ball bearings. Zalman incorporate a heat pipe in most of their units to reduce the need for the fan to operate. Alternatively, Corsair often a liquid cooling add-on (purchased separately) to achieve a similar thing. Periodic cleaning of all units will help keep dust levels down, which helps reduce fan load thus keeping them quieter and cooler.


This is a list of different power supplies that are highly efficient and reliable, and have done well in reviews. It is suggested you do an external search to get more information on each product as unfortunately, external reviews cannot be linked within the guide. The name is linked to a search on ebay for that product. After the name is the figure in Amps that supply provides on the 12v rail, followed by the number of 6 pin PCIe connectors, as required for most mid-range to high-end graphics cards.

20-29A Recommendations
350W Silverstone ST350 25A 0
360W Zalman ZM360B-APS 26A 2
380W Antec EarthWatts 27A 0

30-39A Recommendations
430W Antec NeoHE/NeoPower Modular 32A 1
460W Zalman XM460B-APS 34A 2
450W Corsair VX-450 34A 1
500W Antec NeoHE/NeoPower Modular 38A 2

40-49Amp Recommendations
520W Corsair HX-520 Modular 40A 2
550W Antec True Power Trio 42A 2
500W Zalman ZM500-HP Blue LED Fan, Modular 43A 2
600W Thermaltake ToughPower 48A 2

50-59Amp Recommendations
620W Corsair HX-620 Modular 50A 2
650W Silverstone Olympia OP650 54A 2
650W Silverstone Decathalon DA650
 Modular 54A 2
600W Zalman ZM600-HP
 Blue LED Fan, Modular 54A 2

60-69Amp Recommendations
750W Corsair TX-750 60A 4
750W Thermaltake ToughPower Modular 60A 2
750W Silverstone ST75ZF 60A 4

70-79Amp Recommendations
850W Silverstone Olympia OP850 70A 4
850W Silverstone Decathalon DA850 70A 4
850W Zalman ZM850-HP Blue LED Fan, Modular 74A 4

80-89Amp Recommendations
1KW Corsair HX-1000 Modular 80A 4
1KW Coolermaster Real Power M1000 Modular 80A 4
1KW Coolermaster Real Power Pro 80A 4
1KW Silverstone Olympia OP1000 80A 4

100+Amp Recommendations
1200W Thermaltake Tough Power 100A 6

Recommendations for Ultra Quiet PSUs
Zalman PSUs  Modular
Corsair HX series PSUs Modular
Corsair VX-450 PSU
Antec NeoHE series PSUs Modular
Antec EarthWatts series PSUs

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