Originally Posted by KaRLiToS
850w is not enough for HD Radeon 7970 Tri-Fire.
For overclocking, 1200w would be more appropriate.
He's not overclocking them.
I'll start with two, and then I'll the 3rd: with two 7970s under full in their system, their PSU pulled 567W from the wall outlet. One 7970 alone pulled 195W, so adding that to make 3 makes 567W become 762W. Their CPU was idling during this power consumption test, so I will just add 150W for his CPU under full load which then makes the PSU pulling 912W from the wall outlet. So this means that if the PSU is 90% efficient, then the computer is pulling 820W from the wall outlet. The gaming power draw will be even lower at around 700-800W. A good quality-made 850W would be able to deliver 850W continuously with no problem if it's ever needed.
Besides, I recommended a 1000W PSU and two 1050W PSUs because that's what ended up being recommendable.