Energy calculations don't really work well in an environment where there is energy loss.
Have to disagree with that statement. In fact it would be pretty difficult to do an energy based analysis if the wasn't energy lost!
Everything else being equal, energy based calculations are always easier because you're dealing in scalars not vectors.
Never mind the L/D ratio, CD, CL, which way the forces are pointing.... if you know the cruise thrust, speed and mass of the aircraft the rate of descent is a one line calculation. And the descent angle not much more.
pb