Hands on: Choose the right power supply

Power supplies are one of the most important yet overlooked components in a PC. As PC components grow hungrier, it’s the power supply that needs to keep up, delivering sufficient current and stable voltages to maintain reliability. A poor or under-rated power supply can cause stability issues and ultimately cost more to run.

The potential cost savings of a good power supply, not to mention its influence on stability and handling of upgrades, makes buying a budget model a false economy ­ and a better supply may cost less than you think, with good models costing from just £45. In this Performance column we’ll look at the key specifications behind a decent power supply.

Power
The most obvious power supply specification is its rating in watts, but that’s about as far as many buyers look. A 500W power supply may sound better than a 350W model, but dig a little deeper and you’ll see why this figure doesn’t tell the whole story.

A PC power supply has to convert mains AC into a variety of DC outputs. Modern units deliver three main DC outputs ­ or rails ­ at +12V, +5V and +3.3V. In addition there will be a -12V and +5VSB rail, the latter allowing a PC to come out of standby.

The total output of a power supply is divided between these rails, but in different ratios depending on the quality of the unit. In a modern PC the processor and graphics card both consume 12V, which, as the hungriest components, makes the +12V rail in a power supply the most important.

So you should look for a power supply that delivers the lion’s share of its output on the +12V rail. Lesser models may not. Check the label on the power supply to see the breakdown in amps.

Avoid power supplies that deliver more amps to their +3.3V and +5V rails than they do to the +12V rail. This is an easy way to boost the total wattage figure for marketing purposes, while neglecting the critical +12V rail.

Ideally, the amount of watts a power supply can deliver to the +12V rail should be around three-quarters or more of its total power rating. To find out, simply multiply the number of amps for the +12V rail by 12 and see how many watts you end up with.