Sizing power supplies...things I don't quite get

[Roger]

So let's say you are building a dual gpu computer with a high-end cpu (100W TDP) and your GPUs are 200W TDP each running flat-out.

You have a single HD and maybe an optical drive, so maybe another 50W? So a 550W to 600W power supply should be sufficient.

But if you use a lower power cpu, say 65w TDP and maybe a small flash drive (5w max) shouldn't you be able to drop the size of the power supply accordingly? Yet I see most of the midrange or higher end graphics cards saying they require a minimum 500W power supply.

I guess what I'm wondering is what sort of fudge factor do you need to build in for these systems? Just seems like at most you would need an 800w supply for even something with as many as 3 GPUs and yet I see people on these enthusiast sites recommending 1000w or more.

 

I've thought about adding a 2nd GPU for OpenCL/Cuda stuff but can't think of anything it would help with other than Premier and Photoshop, so not really sure it is worth the added expense and noise (and heat).