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

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