1) Solar Plants should have their output drop off a certain amount, in proportion to the rainfall in a hex that season.
The trick of course would be coming up with the right formula. But the idea here being rainfall = cloud cover = less sunlight = less power. This should encourage solar power construction in logical places like dry deserts, and away from rain forests.
2) Hydro Plants. A new power building, with the same stats as Volcano Plants. Can only be built adjacent to a river of at least 5 hexsides in length, no more than one per river. Make it a fairly early tech, probably Tier I?
3) Speaking of Volcano plants, maybe limit them as well, to no more than 1 per Volcano? Right now you can do 3, which means one volcano is typically all you need.
The idea here with all of the above is to make map terrain features more strategically relevant to power strategy, in general.
1) The Game and AI already has issues with stuff like Farms producing different amounts based on temperature. Making energy-production variable too? Not the best idea.
2) There is no reason you can not have multiple hydro-plants on the same river. 3 hydro power Plants on the same river is nothing special.
Colombia River has 5
Jinsha River has 7
Hexesa are huge at 200km on the short diagonal. The Jinshan clocks in at 2290km lenght.
So 1/hex seems like a fine limit to me. But in turn it should propably have no upgrades.
Of course as the "Grand Ethiopian Renaissanc Dam" shows, this can cause issues for people relying on the water down below. Maybe the water flow can be used to decide how much power it produces?
3) You can built more then 1 Volcano plant per volcano? That sounds like a plain old bug. Propably faulty exlusion rules.