Get a Kill-a-Watt and measure the power usage...
He can do that, or use his UPS software after plugging in any peripherals to another source.
Howsoever that may be, you might apply a factor of 0.9 to the reported value, given PSU efficiency ratings for good units these days. CPU power consumption can be measured without any such estimation through something like HWMonitor.
We discussed this topic on some earlier threads -- some of my own initiation. I've got two GTX 970s in SLI, and it was a matter of concern to me: my processor is overclocked so that maximum load wattage is about 136W. The graphics cards have a stock specification for maximum power-consumption (without OC), but with these cards, figure you can extrapolate to the 110% power limit.
But here's the crucial point of it. There are good PSUs, "so-so" PSUs and s***ty PSUs. You get what you pay for. It is also possible to buy a 1000W PSU that falls into the second or third categories.
I'd uncovered a Hardware Secrets review of a 750W XFX model definitely confirmed in several ways (and by the reviewer) to be a rebadged Seasonic. The testers pushed the wattage draw on the unit to 900W, which it was able to sustain. At something between 900 and 930, the protection circuitry in the PSU kicked in and shut it down.
Not only is it very unlikely that any real-world computer usage will push both SLI'd gfx cards and CPU to their limit of power-consumption, but if the PSU is a good one and seems rated to just equal such a hypothetical maximum, you'll do fine. The maximum wattage ratings for good PSUs are determined as a mean or average power draw, so the spikes that occur for items like the Maxwell cards won't much matter.
The 5960X has a TDP of 140W -- at stock speeds equal to my OC'd SB-K processors. OC'd, it will be higher -- someone else would know -- but I"d guess 200W or more. So for comparison purposes, I'll say that this issue worried me initially with a 650W Seasonic PSU. Enough further investigation of the matter led me to conclude that it was plenty. Plenty, with the 970s clocked to ~1500Mhz and the sig-rig CPU OC. The most I was able to squeeze out in stress-tests on the graphics cards was about 580W x 0.9 or ~530W; the CPU was showing approximately 80W in HWMonitor. If I were to add the addition 60W for full load CPU power-consumption, I would still have room to spare for the Seasonic spec, and probably double that.