a brand new i7 7700k and a 1080ti card will work with a 550W power supply. observe.
https://pcpartpicker.com/list/ 461W
not a real system build (please keep critique to yourself), just an example to show that newer parts are more power efficient than older parts. Barring needing 2 GPU's 550-600W will do the job handily
most can support 2 GPU's