five days you want your hospital to have electricity. you want somebody in an apartment building not to freeze to death. and so taking these intermittent sources and adding them in means that we either need peakers, things that come in supplementing, or we need massive storage. and so we do not have an economic way of converting the energy system to a zero co2 system. only through big innovation in storage. and so getting the costs down. will we have that? that's what we need. and it could come from solar. it could come from nuclear. there's a variety of technologies that might provide that solution. we need to fund thousands of entrepreneurs to drive those costs down. because otherwise, middle-income countries aren't going to pay some huge premium for their energy. >> and do you feel like we are at the cusp of an energy revolution? >> well, i'm hopeful.