That 750W Seasonic will definitely be adequate for the dual 5870's!
If you look carefully at the benchmarks, a more practical load test - World of Warcraft pulls only 430W.
The 664W that was quoted was for a theoretical benchmark (OCCT) that tries to stress the CPU and GPU to 100% -- this is more than what you will realistically see in use.
Realistically, the system with dual 5870's will pull ~160W-220W idle, and in the range of 350-500W at load. It's only with crazy stress test programs that you'll pull 600W+, or if you're both gaming heavily, and encoding something intensive and loading up all of the CPU's 100%. Overclocking while overvolting (increasing CPU voltage) can also increase CPU power pull by 50-100W.
Your link goes to the Corsair AX750.
Anyways, looking at the review for 5870s here:
http://www.anandtech.com/show/2841/26
The test rig with 5870 xfire uses up to 664W (that number should include the PSU's less-than-perfect efficiency, I believe). A 750W PSU should be more than adequate.
That's good thinking, but I think "professional" reviews like Anandtech use measuring devices that measure actual system power consumption, not "at-the-wall" power draw. So a 750W PSU is rated to provide 750W of power to components after conversion of AC (wall power) to DC (computer power). At the wall it would pull more, for example at 80% efficiency a 750W PSU would pull 940W!
I could be wrong on this, but I think that the figure given on AT is measured by voltmeters hooked up to the system, not something like a kill-a-watt, which would measure power consumption at the wall. If someone knows for sure that would be very helpful to know!
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You always want to leave yourself a buffer of ~100W+ from the maximum capacity of the PSU because, as you read PSU reviews you realize that PSU's hit their peak efficiency when they are loaded at usually about 20-80% of the rated capacity (so for example, a 750W PSU gives you peak efficiency when the system is demanding 150-600W).
For your system, I'd say that 750W is definitely a good choice. You won't be pulling more than 450W in 99% of scenarios, and when just surfing the web and doing light tasks, your consumption will be closer to 200W.
isnt there something like you can only use like a % of your total psus rating?
the test system is at 664W under full load.
can i just add up components and get the necessary PSU watts?
also updated the link in my original post.
thanks!
Yes, you can just add up the components to get the necessary watts - the difficulty is finding exact figures for all of the components. I use rough guidelines when choosing my PSU. Generally hard drives pull about 5W each, motherboards from 10-20W, quad core CPU's are usually rated 95W or 130W, RAM takes very little, about 4-5W (usually you can just include that in the motherboard figure), and then check reviews for your Video card's TDP (peak power demand). 5870's are rated for 188W TDP (each), and idle at roughly 30W each.
