Power from power plants, lvl 1 to lvl3, can build them anywhere.
mass from fabricators and mainly mass extractors, on ''mass'' spots on the map.
Usually build 10 or so lvl 1 power gens, then whenever my lvl 1 engineers get idle, I just make em mass build lvl 1 power gens and mass fabricators, 2 power gens per fab, usually end up with 100's of them.
Always try to make next lvl pwer gens though, one lvl 2 power gen is same as 25 lvl 1 power gens, and a lvl 3 pwoer gen is same as 5 lvl 2 power gens...