Understanding power supply units

What do the specs and tests mean?
Once you have an idea of the needs of your components, you need to find a PSU that matches them. All PC power supplies carry a rating, in watts. The obvious conclusion to draw as a consumer is that the greater the wattage, the more powerful the power supply and the more components inside your PC it will power. But, while this is a good rule of thumb, it’s far from the whole story.

Most decent modern PC PSUs come with a rather confusing table of numbers similar to table 2 (see pdf download). The quoted wattage of the PSU is given by the ‘Total power’ figure at the bottom of the table, but above that are various other power outputs which are also crucial to the smooth running of your PC.

As you can see from our example power supply, the manufacturer has been quite explicit about the maximum power that can be delivered on various power ‘rails’ – individual power circuits – at the same time. But some power supplies try to hide, or omit entirely, these combined power limits when describing their capabilities.

For direct current (DC) circuits, power in watts is calculated simply by multiplying volts by amps. In our example, simply totalling up the maximum power available to each rail would have produced a much higher, and completely misleading, total power output of 840.9W. So, looking at a single power figure in watts simply isn’t enough to tell you if the PSU is going to be adequate for your needs.

A modern power supply, meeting one of the ATX specifications, will supply multiple rails, each carrying the different voltages required by different parts of your PC. These are typically 3.3V, 5V and 12V. Different components have different power requirements and may make different demands on some or all of the available rails.

Hard drives, modern CPUs, optical drives and PCI Express graphics cards all use the 12V supply. Many high-end PSUs come with multiple, discrete 12V rails, which allow you to distribute power evenly between all these components. An additional 5V standby rail is also provided to enable some devices to receive power even when your PC is turned ‘off’. This enables functions such as your front power switch to work when you turn on the PC.

Efficiency
Usually specified as a percentage, a PSU’s efficiency determines the proportion of power consumed by the PSU that is converted into useful power in the PC. A PSU with an efficiency of 90 per cent that consumes 500W of power will provide 450W to the PC. The ‘missing’ 50W of power is wasted, mainly as heat, which must be removed by fans or other cooling systems.

The efficiency of a power supply will often decrease alongside the load placed upon it, so a PSU going flat out to feed a fully loaded system may perform more efficiently than when fuelling a less power-hungry PC. You should look for a PSU with a rated efficiency of 80 per cent and above; for example, the Galaxy range from Enermax offers 80-85 per cent efficiency from 20-100 per cent of load.

Many PSUs come without efficiency specifications – don’t be surprised to discover that these can be very inefficient. In our tests, we found efficiencies as low as 46 per cent, which could mean wasting over 500W of power under full load.

Noise and cooling
Almost paradoxically the PSU, while generating heat, also plays a major role in cooling the interior of your PC case. Often fitted with multiple fans, the PSU draws air through the PC case and blows it out the back. In so doing, the PSU cools both itself and the other components.

Fans are the major cause of noise coming from your PC, and many PSUs are designed to minimise the noise produced by selecting quiet fans and running them only as fast as is necessary to keep the system at a pre-determined temperature. As the temperature rises, so will the fan speed and the noise.

If the PSU is efficient enough, it may be able to stop the fan entirely, relying only on its own internal heatsinks under lighter loads. Such power supplies are known as ‘semi-fanless’ and will reduce PSU noise virtually to zero under most conditions. Cheaper models may have user-selectable fan speeds or no fan speed control at all. Even if you’re not concerned about running a ‘silent’ PC, it’s well worth looking for a low-noise power supply, as the differences in noise output can be huge.

Whether you’re upgrading a PC, building a system from scratch, or just trying to get to the bottom of mystery crashes, remember the power supply may just look like a boring box in the corner of your PC’s case, but it’s one of the most important components in there.