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Given that I am building a home computer from scratch and I have several components (MB, CPU, HDD, Video, Sound, etc) which have specific power requirements, is there a formula for me to calculate the size/capacity of the Power Supply I will need? I want to be able to expand the setup later on also...

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up vote 2 down vote accepted

eXtreme Power Supply Calculator Lite

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Add up the wattage on all the labels and round up. According to PC Power & Cooling:

  • AGP Video Card 30W - 50W
  • PCI Express Video 100W - 250W
  • Average PCI Card 5W - 10W
  • DVD/CD 20W - 30W
  • Hard Drive 15W - 30W
  • Case/CPU Fans 3W (ea.)
  • Motherboard (w/o CPU or RAM) 50W - 150W
  • RAM 15W per 1GB
  • Processor 80W - 140W

They also state:

For overall power supply wattage, add the requirement for each device in your system, then multiply by 1.5. (The multiplier takes into account that today's systems draw disproportionally on the +12V output. Furthermore, power supplies are more efficient and reliable when loaded to 30% - 70% of maximum capacity.)
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