First of all, you were incorrect.... It's not 1400, and also not 1300. It is, as i have suggested, 780, as could be easily calculated from the product description
here. Each chip is 400 GH/s, there are 3 chips for 1200 GH/s total. At advertised .65W/GH, that totals to 780W. Any other number you arrive at directly contradicts the company claims.
Try to learn something for once. The efficiency of the chip isn't the entire power requirement of the entire system.
The reported efficiency AT NOMINAL HASHRATE is ~0.65 J/GH. So at 400 GH/s that is 260W @ ~1VDC (actually voltage depends on chip spec and hasn't been reported but likely is 0.7VDC to 1.0VDC). An ATX power supply delivers high current on the 12V rail, however no AISC runs at 12V and thus the 12VDC has the be converted to ~1VDC and conversion means energy "lost" as heat (otherwise the VRMs would be cold to the touch). A quality high current 12 VDC to sub 1 VDC regulator is going to be ~90% efficient so that is 288W @ 12VDC per board. 288W @ 12VDC in = 260W @ ~1VDC out + 28W as heat.
Sill 288W isn't the whole story either. The watercooling loop requires power as well. How much? Not sure but lets guestimate. Each module includes 2 fans likely high RPM so lets go with 12W ea and a pump say another 6W. So 30W per cooling loop plus 288W per board is 318W total. Lastly there are a pair of exhaust fans. Lets guesstimate 12W ea. 318*3 + 12*2 = 978. So 978 not 780 watts per rig.
At nominal hashrate (400 GH/s) & 0.65 J/GH.
Per chip - 260W @ 1VDC
Per board - 288W @ 12VDC
Per board including cooling - 318W @ 12VDC
Entire System including exhaust fans - 978W @ 12VDC
However that is only at nominal 400 GH/s hashrate. Hashfast has indicated they believe the chips can be pushed harder. How hard? Well I certainly don't know and HashFast won't even know for sure until they get the final silicon but lets say they can run at 450 GH/s at nominal voltage and with a 10% overvolt can be pushed to 500 GH/s. In Silicon devices, power consumption increases by the square of the voltage increase. So 10% higher voltage = 21% higher power or 0.78 J/GH.
500 GH/s @ 0.78 J/GH (AS AN EXAMPLE ONLY)
Per chip - 390W @ ~1VDC
Per board - 433W @ 12VDC
Per board including cooling - 463W @ 12VDC
Entire System including exhaust fans - 1,413W @ 12VDC
So designing around a 780W power supply would limit the potential of the device. Maybe 500 GH/s per chip isn't possible but maybe 440 GH/s or 465 GH/s is. HashFast stated there were no power supplies in the range they were looking at (1300W to 1400W DC) from vendors they were interested in. Also stated was that even if one was available it was more expensive than two smaller units.
SeaSonic's "X series" for example maxes out at 1250W and a 1250W unit has higher price than a pair of 750W units (1500W total).