Power Requirements
Modern graphics processors are very complex; the newest flagship models contain more than 200 million transistors. Currently, the record for most transistors in a consumer graphics chip is held by NVIDIA's GeForce 6800 GT and Ultra chips, which weigh in at 220 million transistors. To give you a frame of reference, bear in mind that even Intel's Pentium 4 EE CPU consists of "only" 178 million transistors - 149 million of which make up the second level cache!
The hunger for power of modern graphics cards is correspondingly high, and can no longer be satisfied through the current supplied through the AGP slot alone. While the AGP slot supplies up to 45 watts, a GeForce 6800 Ultra draws up to 110 watts under full load. To make up for the difference, 2 additional ATX Molex connectors need to be plugged into the card. This is an extreme example, and most cards need only one such auxiliary power plug. The new PCI Express interface improves this situation by offering up to 75 watts, but even this obviously isn't enough for the highest-end cards.
The way the auxiliary power cables are split up is important. Whenever possible, the graphics card should always have its own, unshared cable. The only other component that can be attached to the same cable without risking trouble is a case fan.
Depending on the power requirements of a card, the power supplied by the motherboard may not be enough. In this case, cards require auxiliary power connectors fed directly by the system's power supply. This picture shows the types of connectors currently in use.
In 2D mode, graphics cards draw comparatively little power. However, in a 3D game that puts a lot of stress on the graphics card, the CPU and potentially even the hard drive, the power draw can peak quite suddenly and overwhelm the PSU. The unavoidable consequence is a crash of the entire system.
As a reaction to the power needs of their cards, both ATI and NVIDIA state minimum requirements for power supplies. However, these should only be considered guidelines, at best, since the power supply also needs to power the CPU, the drives and every other component in the system. The power requirements quoted by the manufacturers refer to a standard PC with a modern CPU, say a Pentium 4 3.2 GHz, a hard drive, a DVD drive and a soundcard. If your computer houses more components, it's better to play it safe, bite the bullet and buy a brawnier PSU.
Also, it should be noted that a good 350 watt power supply can deliver much more stable voltages than a cheap 450 watt model. The specifications of the PSU, such as the amperage at a certain voltage, can be of great help in making an educated buying decision. Such information can usually be found on the manufacturer's website. If the manufacturer doesn't publish any information, it's usually safer to choose a different model. You can find more information on power supplies in this article: Getting the Right Power: 15 PC Power Supply Units .
Power requirements of the newest high-end models:
- X800 XT PE: min. 350 watts and one additional connector on an unshared cable
- X800 Pro, 9800 XT: min. 300 watts and one additional connector
- GeForce 6800 Ultra: min. 350 watts and two additional connectors; cable can be shared. 480 watts required for overclocking, with two unshared cables.
- GeForce 6800 GT und 6800: min. 300 watts. One unshared cable from the power supply.
Smaller models are less demanding where the power supply is concerned. A standard 300 Watt power supply will usually be sufficient, at least as long as there aren't too many other components in the system.
