those are 300w gpu's. add in the rest of the system and you're easily at 800w or more. to leave some headroom and such 950w or more is what you need. those are some power hungry cards!!
Unlikely to last long. I'd go with atleast 850w, but depending on the rest of the hardware you may need a larger unit.
https://www.guru3d.com/articles_pages/powercolor_radeon_r9_390_pcs_8gb_review,8.html