Energy and water.

Update, 2017-07-19: The Solver

Elon Musk at the National Governors Association conference last week:

• It is possible to supply every electron needed to keep America humming by covering just 100 square miles with solar panels.
• “The batteries you need to store the energy, to make sure you have 24/7 power, is 1 mile by 1 mile. One square mile. That’s it.”
• “People talk about fusion and all that, but the sun is a giant fusion reactor in the sky. It’s really reliable. It comes up every day.”
• “We’ll need to be a combination of utility-scale solar and rooftop solar, combined with wind, geothermal, hydro, probably some nuclear for a while, in order to transition to a sustainable situation”
• “People do not like transmission lines going through their neighborhood, they really don’t like that, and I agree”
• “Rooftop solar, utility solar — that’s really going to be a solution from the physics standpoint. I can’t really see another way to really do it.”

You can watch the keynote below: