The U.S. is more reliant on solar power than ever. When an eclipse blocks out the sun, we get a glimpse of how our grid uses fossil fuels to compensate — and a future where it won't have to.

On April 8, millions of glasses-clad onlookers will, for the second time in seven years, hold their collective breath. As the celestial odds align, the Earth and moon will be in the perfect position to blot out the sun across the U.S., along with the solar power that makes up an increasing share of our energy mix. With eclipses anticipated decades in advance, local utilities have had time to prepare for the big day. From little Vermont to hulking Texas, how the eclipse will impact the energy grid paints a picture of energy progress, but also how we still depend on fossil fuels to stay resilient.