If you are installing a new graphics card, you may be considering upgrading the Power Supply Unit (PSU) of your computer to support the card.
Graphic card manufacturers provide recommended minimum power supplies. These minimums already include their assumption of an average computer’s power requirements for motherboard, CPU, RAM, drives and fans. So when the stated minimum is 400W for example, it doesn’t mean that graphic card takes 400W, but that the card takes perhaps 150W and they have allowed 250W for everything else.
Many pre-built machines, from Dell, HP, Gateway and others, come with the absolute minimum power supply that will run the machine – from 180W to 250W. These are inadequate for powering an upgraded video card, so the power supply will have to be replaced.
When replacing a power supply, you need to look at three things
- Physical compatibility
- Output power
- Connectors
Physical compatibility is whether the power supply will fit the case and match up with the mounting holes and fan vents. Most tower computers have ATX standard power supplies, so the mounting holes and basic size will be standard, and any ATX power supply should work. Watch out for machines whose PCI-e cards or CPU cooling are close to the back of the power supply. Some ATX power supplies are longer than others front to back, and may conflict. If you have a machine with a tight layout, look for a short ATX PSU.
Measure the power supply size visible on the back of your machine. It should be approximately 6 inches by 3 1/2 inches if it is a standard ATX. http://en.wikipedia.org/wiki/ATX#Power_supply
If you have a small form factor, slimline or portable desktop machine, you may not have an ATX standard PSU. This makes it much more difficult to find a suitable replacement PSU, and you may want to consider replacing rather than upgrading the machine. Servers and rack-mounted machines use some different power supply standards such as EPS and FlexATX.
Power supply form factors http://www.tomshardware.com/reviews/power-supply-specifications-atx-reference,3061-4.html
Output power we have touched on already. The output of the PSU needs to be higher than the peak requirements of all of your components together, plus at least a 10% buffer for safety. More capacity in the power supply is never bad. Keep in mind that the advertised output power is not always the actual available power, budget brands are somewhat notorious for over-advertising their product. Well known brands are better at meeting their specs, and with reliable, clean power.
This website helps you calculate how many Watts you need http://extreme.outervision.com/psucalculatorlite.jsp
Power supplies also have efficiency ratings, which measure the amount of power that is delivered to the PC vs. the amount that is lost to heat and inefficiency. The 80PLUS ratings are for power supplies that are 80% efficient or better, there are Bronze, Silver, Gold and Platinum levels. The Gold rating means the PSU will be at least 87% efficient at 100% output http://en.wikipedia.org/wiki/80_PLUS
There are differences between the Amps available on each output rail, but that is outside the scope of this article. Google for many debates on PSU specs, rail design and quality. http://www.tomshardware.com/reviews/power-supply-specifications-atx-reference,3061-2.html
Connections: the power supply has to have the appropriate connectors for your motherboard and peripherals. The standard ATX motherboard connector is a 20+4 or 24 pin connector. Then there is an additional 4 pin or 8 pin ATX12V connector to supply regulated power to the CPU. For hard drives, you will need some SATA power connectors, and at least one 4 pin Molex peripheral connector for older drives and devices. A 4 pin floppy drive connector is usually included but less commonly used.
The connections for the video card(s) are important, they have to match what your video card requires. These are called PCI-e Graphics (PEG) connectors, and carry 75 W each to the video card. There are 6 pin and 8 pin variations. Many video cards require 2 PEG connectors, so it you are installing a pair of video cards in SLI or Crossfire mode, then you will need four PEG connectors. Look for a power supply that is SLI and or Crossfire certified if you are planning to use dual cards.
PEG power http://www.tomshardware.com/reviews/power-supply-specifications-atx-reference,3061-12.html
If you have a machine with dual CPUs, such as a Xeon server motherboard, then you may need an additional 4 pin or 8 pin ATX12V CPU power connector
One final thing to look at is noise. A well built, efficient power supply with a large diameter 120mm or 140 mm fan will be quieter than a cheaper, hotter PSU with 80mm fans. Some PSUs give sound ratings, but there aren’t any industry standards for reporting noise levels.
Reviews and ratings
Keep in mind that while there are a large number of brand names in the Power supply market, relatively few of them actually design and build their own products. Most of the building is done by contract manufacturers, and the design make be done in house or licensed from others. For this reason, you cannot say that Brand X is better than Brand Y, you have to look at the individual models, or model lines, and research who makes them and what the reviews, owner feedback and testing reveal.
http://www.anandtech.com/tag/psu