Assuming you have basic components installed in your PC, a good quality 550W is a recommended PSU wattage for the GTX 1080. It has more than enough juice to power the rig while still maintaining good headroom/clearance for OC'ing and other future basic upgrades (excluding multi-GPU setups).
Highly-recommended PSUs of that range are: Seasonic G-550, EVGA SuperNova G2 550, EVGA SuperNova G3 550, and Corsair RM550x.