How can your rig only consume 750W?
Each card has TDP of 120W which could reach up to 180W of power in real usage.
Also the processor, motherboard, Drives and peripherals consumes power, even with 120W the whole rig would consume more than 750W.

You might be losing more than you think, how much is your electricity cost?

"Free Electricity" Dudes.
Huge Mining farms with low-cost Electricity or additional alternative sources (Ex. Solar).