How can i calculate what PSU i need in order to run my entire rig? What should i consider in order to find the right one? and how to be sure that the result will be scalable?

I've a very low bugdet, so i would like to start with just 1 GPU and then add the others as soon as i make some profit.


What PSU should i consider for 1x rx 580? or 1x gtx 1080?
If i buy a PSU with more watt than i need, does it consume more or just take "what it needs"?
This are very noob question, hope you can help me.. Thanks