The existence of dark energy itself is hypothetical, so it would be impossible to quantify the energy properties of dark energy.
With regard to your question of the expansion of the universe, I believe any answer would rely upon supposing the very structure and form of the universe itself, there are theories that would argue the rate of expansion slows down because of gravitational forces acting upon the universe itself. At some point the universe would begin to contract upon itself much like a star and collapse, and the big bang cycle would start again.
Energy would be lost however and each subsequent collapse would yield a smaller universe, assuming the universe isn't infinite.