I think there will be wind energy (unless the sun becomes to hot, somehow makes all areas of earth same temperature and stops wind), solar energy (might make up for the wind energy loss). They might also mix those, make wind farms with solar energy charging thingies on the wind mill (the thingie that collects wind)
Hydroelectric is another obvious one.

At the rate were going right now, our technological 'golden age' has come. Maybe in 20 years we could start sending massive amounts of solar energy from space if thats possible.
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Wind energy is already being used. If you drive through Wyoming, you can see hundreds of these massive windmill things, creating energy.
Solar energy is already being used also, but isn't a very reliable source (Waaaaaaay to expensive)
Hydroelectric is good (that's how i'm getting my power right now), but it just doesn't support larger cities.
I believe Fusion would be the best, but we cannot harnes it yet, so we are doomed