I used to have to rely on the energy app, way back in 2014, before they added the energy estimate into the Navigation screen. That was a little bit of a pain having to keep changing screens, because I wanted to keep Navigation up, plus music, but I would have to keep switching one of them every once in a while to check on the energy estimate.
Now that it shows the remaining battery % at the bottom of your navigation directions, you don't need the energy app anymore. I just keep an eye on that % number in Nav and try not to let it get below 15% or low teens for most of the trip. So I may need to turn my cruise control down one or two miles per hour and then watch that percent number for a few minutes if it's dropping below my comfort level some.