How to calculate PSU requirements

[Zerikin Loukbel]

I've been reading lots of information on various PSUs but I still have not found a reliable method to determine the power needs of a system. I did find http://www.extreme.outervision.com/powercalc.jsp but it gives a suggestion in watts and from what I have read its really about the amps.

Currently I am looking to be getting some sort of quad core with air OC and a 8800GT or equivalent. In general though what is a good way to determine the power needs of a system?

 

[Redbeard]

I've been reading lots of information on various PSUs but I still have not found a reliable method to determine the power needs of a system. I did find http://www.extreme.outervision.com/powercalc.jsp but it gives a suggestion in watts and from what I have read its really about the amps.

Currently I am looking to be getting some sort of quad core with air OC and a 8800GT or equivalent. In general though what is a good way to determine the power needs of a system?

The amps are important, but all the wattage concerns you have should be about 85% or so on the +12V rail, which is really the only one to worry about these days.

Look up your processor's TDP on the manufacturer's web site. For example, Wikipedia has a list of the TDPs for Intel processors HERE.

This will give you the wattage for your CPU.

Add a base of about 50-70W for motherboard, memory, fans, optical drives, etc.

Add another 15W per hard drive.

Find your video card's power draw through google results or reviews. The 8800GT uses about 110W or so.

Add all that up, assuming a Q6600, 8800GT, 4 hard drives, and a typical system, you'd be around 350W or so, real power under max load, and maybe 1/3 of that at idle. Since your PSU should never be loaded more than 3/4 or so, ideally you'd want about a 450-550W PSU.

Out of that 350W, about 85% of it, or around 300W, will be on the +12V rail. That's about 25A. Using our "at max 3/4 of load" theory, you should get a PSU with about 33-34A on the +12V rail, minimum.

A good 550W PSU will have at least 40A on the +12V rail.