Metal, Energy, and Build Time

I would not necessarily make power the same globally. For instance a tidally locked planet would have high solar power on one side and little to none on the other. Wind power could vary with elevation. Some power resources would only be available in specific locations, like geothermal is only available near an active volcano or something.

And even other planets could have less solar power on the poles than on the equator.

There are also peaks of eternal light, which (unsurprisingly) have eternal light, which would allow a solar to always collect decent power (assuming that the planet has any decent solar power)

I think planet rotation for solar may be a good, realistic idea that is going to significantly intrude on gameplay. Suppose that we have half a planet be lit up, and the other half is dark because it is on the night side. Either we have to have the planet rotate quickly, because otherwise one half will be dark for the duration of the game, or we have to let half the planet simply not be usable for solar in the game's timescale.

Differentiating entire planets by solar quantity is simple, and interesting. Differentiating the surface of a rotating planet in a continuous distribution depending on solar intensity is becoming overly complex for decreasing gains for gameplay.

Tidally locked planets, which don't rotate, and the same land surface will always be in sunlight, avoid this complexity problem. However they introduce minor (solvable) gameplay problems. For example, it would be unfair to have one player spawn on the light side, and the other on the dark side. Not necessarily game-breaking, and also avoidable, but important to note.

I think we should skip variable output solars from rotating planets, but I can see tidally locked planets working, where a fixed area is viable for solar, and a fixed other area is not.