practically speaking though, most of our energy would need to come from either fission reactors, hydroelectric dams or solar heat. Wind energy may be practical in some outlying areas, but they don't scale well for providing a city with power.
The problem with fission energy is that such reactors aren't cost effective. In fact, no such reactor to date has ever made a profit. They simply don't work because literally capitalism.
The problem with hydroelectric dams is that they require a river, and not every river should be dammed. They also cannot be built in places that are affected by earthquakes.
The problem with solar heat is that it needs a large area for all the mirrors and produces fairly little power for the area it needs. Also no power at night.