[Eclipse]

There really is no other forms other then Solar, Wind, Water, Grid and Generators.

Even the US government in places like Cheyenne Mountain, Mount Weather, Raven Rock, and other facilities use diesel generators, I bet they use solar too.

Nuclear would work, but it's just a tad too expensive.

Geothermal or hydro(via underground river?). Only useful thing I learned from Geography.

It'd have to be placed specifically near a geothermal vent, and it'd be very expensive to harness the energy. With something like nuclear, you only need a small amount of fuel and you get a very large amount of energy.