It's all very well upgrading your graphics card, but your PC also needs enough power to ensure it can deliver the superior video-handling you expect. PC Advisor explains the crucial role of the PSU and tests five powerful units.

At the heart of every PC lies an often overlooked component: the power supply unit (PSU). Rather like a human heart, the PSU gives life to every part of the PC. It's also frequently the cause of a number of system failures.

Inadequate PSUs can cause seemingly random crashes, unexpected shutdowns and even small explosions. It's not uncommon to waste several days trying to heal a constantly crashing system by cleaning out the operating system (OS) and software, only to find that a new PSU cures the problem. As the demands placed on the PSU increase, the voltages it supplies can start to decrease, causing components to fail.

The PSUs shipped in most PCs will allow for minor upgrades, but push yours too far and that 350W unit is unlikely to cope with the extra burden.

High-end graphics cards can demand huge amounts of power, which will double if you're intending to move to a dual-card solution. Both nVidia and ATI certify PSUs for use with multicard solutions.

It's not just graphics cards that can overload your PC, either. If you upgrade your PC with a processor or supplementary hard drives, this can also place an additional load on your PSU.

If you're considering overclocking your machine, a good-quality PSU is crucial. When you're pushing your system to the limit, you'll need your PSU to be able to deliver not only large amounts of power but also the correct voltages you need to get the most out of your machine's new components.

Efficiency drive: A new PSU can save you money in the long run. For every watt of power your PSU supplies to your PC, a greater amount of power is drawn from the mains (the remainder is lost mainly as heat). An efficient PSU will waste less energy and cost less to run. Producing less heat also means that your system will run cooler and fans will run more slowly, resulting in a quieter system.

All the PSUs tested here are 80 Plus-certified, denoting an efficiency of at least 80 percent. Find out more at 80plus.org.

Noise issues: The amount of noise created by a PSU varies considerably from model to model. The companies that specify their sound output will do so in decibels, and anything around 21dB is going to be almost silent. If a company doesn't supply this information then, as always, the web is your best friend.

Connectors: Not all PSUs will be compatible with your particular setup. Modern graphics cards, for example, often require two power connectors: one with six pins; another with eight. Older PSUs with six-pin connectors won't be able to power these cards without the use of adaptor cables, which aren't recommended for long-term use.

On the other hand, too many connectors can fill up your system case with messy, redundant cabling, which will reduce airflow and potential upgrade room. PSUs with modular cabling allow you to install only the cables you need.

NEXT PAGE: how we test

  1. The best 5 PSUs for your PC ranked and rated
  2. How we test power supply units
  3. Power supply unit reviews

It's all very well upgrading your graphics card, but your PC also needs enough power to ensure it can deliver the superior video-handling you expect. PC Advisor explains the crucial role of the PSU and tests five powerful units.

How we test

The PC Advisor Test Centre upped sticks to FSP's testing facility in St Albans for the purpose of this group test. Using a Chroma 61604 programmable 230V AC power source and a bank of Chroma 63103 load modules, we were able to precisely measure the amount of mains power supplied first to the PSUs, and then from the PSU to the PC.

The PSUs were connected to the bank of load modules via a board filled with typical PC power connectors. The load modules were able to simulate the power requirements of various computer components, and their controls allowed us to select the precise power load we wanted to place on the PSU.

We set up the load modules to simulate the power requirements of four hypothetical PCs: the first drawing 350W, the second 700W, the third 750W and, finally, 800W. We tested the PSUs under each of these scenarios, starting with the lightest and finishing with the heaviest load.

For each of these tests, we were able to calculate the efficiency of any given PSU by comparing the power it drew from the AC source with the amount of power that it delivered to the PC. The perfect PSU should be able to convert 100 percent of the power drawn from the AC source into useful PC power but, in a real-world scenario, 80 percent efficiency is a good result.

A PSU needs to be able to provide power at three voltages: 12V, 5V and 3.3V. As the PSU reaches the limit of its capabilities, these voltages can start to tail off. Some tolerance is allowed, but the closer it keeps to your various components' specifications, the better your chances of having a smooth-running PC. If these voltages drop too far, your PC components will receive inadequate power, causing crashes and system failures.

Although the PSUs in this group test have various power ratings from 700W up to 780W, we tested each of them at 800W to see how well they coped with increasing loads. None of the PSUs failed or exploded during testing, which is one possible outcome of such an overload.

NEXT PAGE: PSU reviews

  1. The best 5 PSUs for your PC ranked and rated
  2. How we test power supply units
  3. Power supply unit reviews

It's all very well upgrading your graphics card, but your PC also needs enough power to ensure it can deliver the superior video-handling you expect. PC Advisor explains the crucial role of the PSU and tests five powerful units.

PSU reviews:

  1. The best 5 PSUs for your PC ranked and rated
  2. How we test power supply units
  3. Power supply unit reviews