"cost effective investments in overdrive..." if you want to place some energy generating structure, it shows you how many minutes it will take to pay it's own cost back, assuming all your metal extractors are gridded. Given enough time, all energy structures pay themselves back, but it might take a long time if you are playing on a low metal map or if you already have a crazy high ovedrive. The more expensive the energy structure, the more efficient it is - singularity reactors are best. Wind generators are quite efficient too if the wind is strong at the place where you want to build it (you can see wind strenght when you're placing the windgen).
Can't answer your next two questions.
