I need energy, and have been avoiding the "eco" upgrades... which is better to upgrade, Aqueducts to Hydro Plant, or Pasture to Solar Farm?
For very immediate purposes, Solar Farms are cheaper on your titanium, have more value for the cost, and are more easily upgraded than Hydro Plants. They also replace pastures, which in many cases has the lesser effect on your Catnip production, compared to replacing Aqueducts.
That said, they simply don't give very much energy, only 2 apiece, so you will probably build lots quickly. They have much more value when upgraded (3 energy instead of 2, doesn't sound like much but when you have a lot...), but once you hit 10 or so (more with Metaphysics), the costs start getting high enough that they will equal out with Hydro Plants on energy per titanium spent.
Reactors are really what you want to build, with all the benefits they can give in addition to energy. For energy per titanium they are initially better than Hydro Plants as well, it's just that they not only cost titanium, but hard to craft blueprints as well. If you've already down that path and the costs are too high, you may as well start expanding your energy income with Solar Farms.
Solar Farm. Aqueducts will always keep your kittens fed (you're catnip will stay positive). Once you get into space you can build Hydraulic Fracturer to make oil and turn off your Bio Labs (they still give the science bonus). Satellites also generate energy as will reactors that are buffed by steamworks and magnetos.