The idle power is 25W, the full load power is 320W, so simplistically, we think the mining uses about 295W, or 147W each card.
Er, no. The system less the gpu's is not at 25W when mining; the PSU is operating at a much higher load, there are always inefficiencies (most are only 80-85%), and I can't monitor the PSU I/O by itself, only calculate it. Also, there are case fans ramped up adding to system load, each draws at least 5-10W running at 60% or so. Add it all up and the non-gpu-related draw is much higher, I add it up to about 60W.I think it is similar efficiency. the difference is very small. 7970 is 3-4 year's old card.
7970 20MH/140 = 0.142.
970: 20.75/147 = 0.141.
Bad math; it's 20.75/115 = .18, but I'd rather do it the other way (W/MH). We're only looking at the card power here, but you're including system power in your figure and don't appear to be doing the same with your 7970...7970 20MH/140 = 0.142.
970: 20.75/147 = 0.141.
Let's try to keep our numbers in perspective. I'm not advocating buying GTX cards over AMD for obvious reasons, just saying that a state of the art gaming PC can mine pretty well and efficiently.
But that extract fan power consumption is due to the mining as well. So we have to take that in to account.