I have a Sapphire Nitro+ RX 570 4GB. While dual mining UBQ+DCR, GPU-Z shows 140W plus add another 50W for the riser, so ~190 W total @ 1150 MHz core clock, 2150 MHz memory clock with a -96 mV undervolt and -10% Power Limit.
If you plan on dual mining, for 7 cards I would go with a 1600 W PSU. For a dual PSU I would go with a 1200 W + a 750 W or two 1000 W.
Risers power consumption is calculated in GPUZ.
GPUZ Power = power from the PSU + power from the pcie port (riser)