How to choose the best power supply for a computer

Selecting a new computer PSU or replacing a computer ATX power supply?

This guide will tell you all you need to know to select the best power supply for a computer whether you are building from new, replacing a faulty power supply or just upgrading your power supply to increase the power capability.

These beasts supply the power to all the other components in your PC system and are capable of doing a lot of expensive damage to your components if they should fail.

It would be wise to choose carefully and spend a little more maybe on the best power supply you can afford.

You will want a unit that is reliable, quiet and efficient with enough oomph to cope with your present system and any planned future upgrades.

The hungriest of processors and graphics cards will need something a little more powerful and you will first need to ascertain the approximate load that your computer PSU will experience.

ATX Computer Power Supply

Your computer will most likely require an ATX power supply. There are several types of power supply on the market but the most common is the ATX power supply.

This will be determined by your PC case size which will state if it is ATX form factor compatible.

There are also micro ATX power supplys and mini ATX power supply formats.

Small PCs in Mini-ITX or Shuttle cases require special PSUs with maximum output power of approximately 300W.

A full size computer ATX power supply would be physically much too big to fit these case formats.

Replacing a Power Supply

Computer power supply problems do occur from time to time and if you are replacing a faulty power supply and you haven’t made any other changes to your PC setup then you can examine the old PSU for details of output power and connector types.

The computer output power is measured in Watts (W).

A typical PC will run happily with a 300W to 500W PSU but gaming computers will generally need upwards of 500W.

Hey, there are power supplies out there offering 1000W or more!

These will really only be required if you have a really fast quad processor say with SLi or Crossfire configured multiple graphics cards.

SLi and Crossfire

If you are planning on running multiple graphics cards in SLi or Crossfire configuration then you will need to consider carefully the power requirement and you will also need to check that the PSU has enough PCI-Express power connectors to supply all cards.

Check your power supply is certified SLi or Crossfire compliant and total up the graphic card power requirements plus your processor, hard drive, optical drives, motherboard etc to obtain an estimate of the required power in Watts.

Now add on a further 100W overhead to ensure that your power supply has enough headroom when it is working flat out.

nVidia provide guidance for choosing a power supply for their SLi configured cards at

Silent PSU

A silent PSU can be obtained if you require one for a Home Theatre or Media Centre PC for instance.

Power supplies can make a lot of noise with their fans and if you find this noise annoying or intolerable then you can buy power supplies that have noise deadening features such as large slow running fans or fans with externally adjustable fan speed or automatic fan speed controllers that regulate the fan speed dependant upon use or loading.

Generally, the bigger the fan diameter the better it can move air at slow speeds theoretically meaning it should run quieter.

Some PSUs are available with no fans but are passively cooled with just the internal heat sinks.

Although these are completely silent they tend to run hotter than a fan cooled power supply and are therefore maybe not so reliable.

Our advice? Stick with a fan cooled type.

The Zalman ZM600-HP is a 600W power supply that offers very quiet performance with its fan and heat pipe cooling system.

Modular Power Supply

Typical power supplies have their wires and connectors permanently connected to the internal workings of the PSU and it is difficult to make the wiring appear neat and tidy.

Plastic cable ties help with this but you may wish to consider one of the modular power supply offerings available.

Modular power supplies enable you to disconnect any unused cables allowing for a much tidier solution but they generally cost a little more.


You will need to check that the PSU you choose has enough connections for peripherals such as, SATA, floppy drive, fans, optical drives, graphics cards, SLI / Crossfire etc.

New motherboards use a 24 pin main power connector but older boards may use a 20 pin connector. Check before buying.


Look for a high efficiency or specified efficiency of 80% or better denoted by the ‘80Plus’ logo.

Better efficiency means cooler running and cooler running generally means better reliability.


For reliability, look for a manufacturer who is prepared to offer a 3 – 5 year guarantee.

A longer warranty won’t guarantee a more reliable power supply but at least it gives you confidence that they are prepared to offer this plus a return unit if it does fail.


Take a look at the following companies for quality products.

Antec, Corsair, Cooler Master, Thermaltake, Tagan, Hiper, Zalman and Be Quiet!

For a general budget PC look at the Hiper 530W power supply or OCZ ModXStream Pro 500W.

For a silent PSU you could look at the Zalman ZM600-HP or Be Quiet! Dark Pro 650W.

For a gaming system PSU look at the Thermaltake 850W Modular or Corsair 850W CMPSU-850TX.

Good luck choosing the best power supply for your system.

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