Ha, I had the same idea too and got the dev kit to try and make them but know nothing about making mods for the unreal engine.
I am still trying to figure out what is possible with the dev kit as the last mods I did were back in the days of Quake.
For the logic I was thinking the simplest way would be that solar only creates power during the day.
For powering at night I was going to see if possible to set a generator to auto start if no power from other source.
I was also thinking about doing wind turbines too but though the animated model would be a little too complex for my first mod.