A power supply unit (PSU) is one of the most critical components in your computer case. If the CPU is the brain, the PSU is the heart and if the heart fails, the system dies. Whether you are building a new computer, replacing a dead PSU, or just upgrading an underpowered one, there are several factors to consider when deciding which one to buy.
- Physical Dimensions: Most power supplies and cases are designed around the ATX form factor. However, some smaller cases require one of the smaller form factors such as MiniATX, MicroATX, or FlexATX. Look at the specs of the PSU being replaced or the case to see which form factor you need.
- Connectors: For many years, all motherboards used the 20-pin power connector. However the power requirements of the new PCI-E slots have given birth to a new 24-pin connector. Although adaptors exist to allow you to plug 20-pin PSUs into 24-pin connectors, doing so will put unnecessary strain on the power supply.
- Cooling: Most PSUs have adequate cooling already. Some have dual fans, drawing air in from the PC to keep the computer cooler. Some argue that the incoming warm air puts strain on the PSU components, but there is no clear evidence whether one fan or two is ultimately better for your system.
- Noise: The PSU fans are likely to be the loudest component in your system, and since the PSU is mounted directly to the case, the fan vibrations are often even louder as the entire case vibrates. How much noise is too much noise is a matter of personal preference and the PC role. PSUs can be found with quieter fans or even no fans, although these units tend to run at a higher temperature and/or are more expensive. Reviews of quiet PSUs can be found at sites such as Silent PC Review.
- Dual Video Cards: If you are planning to run dual video cards, make sure the PSU is rated as SLI Certified (for NVIDIA cards) or ATI CrossFire Ready (for ATI cards).
The most important decision is how powerful a PSU your system needs. An underpowered system can suffer from elusive hardware problems and component failure. An overpowered system just wastes your money and generates unnecessary heat. So how big is big enough?
- The Very Simple Method: Typical off-the-shelf computer systems will do just fine with a 350W PSU. Note that most off-the-shelf systems ship with much lower wattage PSUs in an effort to increase profit margins. A more powerful system might require 450W or higher.
- The Less Simple Method: Start with 100W. Add 50W for each hard drive, CD or DVD drive. Add 50-100W for each video card and CPU (higher power consumption for higher end components). Round memory up to the nearest GB and add 50W per GB. So a typical budget system would be 100W + 50W (hard drive) + 50W (DVD drive) + 50W (video card) + 50W (CPU) + 50W (512MB memory, rounded up to 1 GB) = 350W.
- The Complicated Method: Power supplies don't put out a single rail of power, they put out three: 3.3v, 5v, and 12v. Tally every single component, right down to the keyboard and each fan, and list its current requirements in amps for each of the three rails. Total the requirements of each rail and add 25% to compensate for PSU efficiency and future expansion. Find a power supply that can provide this current to each rail.
Once you have all the requirements for the PSU, now is the time to hit the internet and look for reviews of specific models. Stay away from generic or budget brands and stick with quality manufacturers like Antec, Enermax, ThermalTake, or PC Power & Cooling. Low quality PSUs may provide fluctuating, inefficient power that can damage system components. Saving twenty dollars now might cost you hundreds of dollars in failed components a year from now.
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