Green Energy

Either electro-chemically in batteries, but they might not get energy dense enough for seasonal storage.

Or chemically by creating Hydrogen, Methane or Ammonia. Those are very energy dense and except for Hydrogen easy to store with existing infrastructure. Currently it is just prohibitively expensive, because the net efficiency is bad, especially for Methane and Ammonia.

So without an Oracle I can't say which it will be, but one or more of those will work if the world is committed to net zero.