As well polished this game is, managing the amount of power you have is really not that well thought out. Or rather, it lacks a good interface.
First of all, I can't directly see how many Mark III power plants I've built on the galaxy. Or rather, I can't see at all how many power plants I've built all in all.
So say, I have a 200 000 excess power, and I remember I had a Mark III power plant made somewhere. I have to manually skim through each planet on the galaxy view to see which one has 3 energy reactors. And then, I have to pick energy reactors from the list, pick Mark III energy reactor and put stop in it. Now I have 110 000 Excess power. I then need to manually turn off many of some of the mark II energy reactors which is really tedious micromanagement that I'd rather use for something else.
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Just make a simple interface from the CTRLS view where you'll be able to turn off Energy reactors at will without the need to manually visit each and every planet.