No, youre right. A PSUs wattage is the converted output, and depending on the efficiency rating it will draw a different amount of juice from the socket. A Gold rated PSU will have a 12% spill at 100% capacity. So when you are pulling roughly 850W from the wall you would be at its max delivery. I would however not suggest you do this, i would pull the brakes when the killawat reads just above 750W which would put you at roughly 90%... which is fairly high as well.