How to buld power? nice question, that depend only on your microing skills, and what you like to do in game, some players spam a lot of air with t1 power, i mean for them fac 3 pgend fac 3 pgens may be not an option, some one going for ACU upgraide with only t1 power, some one never do that
some one make so low power build that they do not need power spam, and can handle with trees xD
For example i almost never OC t1 units (5000 power for thouse 2-4 tanks? no thanks i better reclaim them after they die)
i prefere do not waste power, and do not waste power to oc t1... (i know some one will disagree ofc)
you can just make line of power, line of templates, squares of power where you can put air fac later, or connect each pgen to 3-4 factories or mex, IMO that only depends on your micro and habits.
https://www.youtube.com/watch?v=6fhjUrSRFGU^Here i am trying to do my usual stuff with no lines of factories and lines of power, but after 2:33 i am microing units while ACU is idle and resoses are stored too much and i decide that "looks like i need land spam and power spam" and after few seconds i can pay my attansion to other stuff