"When I say "an energy miracle," I mean that there will be some form of energy whose 24 hour cost really is competitive with hydrocarbons given, say, 20 years of learning curve. You invent it, then you look at how much its costs go down over the next 20 years, that it really beats hydrocarbons."

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The most straightforward path would be if we could bring the cost of solar electric and wind down by another factor of say, three, and then have some miraculous storage solution, so that not only over the 24-hour day but over long periods of time where the wind doesn't blow, you have reliable energy. That's a path. But energy storage is hard. That's not a guaranteed path.

— Bill Gates