D'Alembert's principle deals with forces and states basically that in a balanced system the algebraic sum of the forces is ZERO.
The law of conservation of energy deals with energy and states basically that in any closed system the total amount of energy is constant ie energy lost is ZERO.
Both of these laws/principles can be used to form an equation considering all the forces/energy equal to ZERO and solved.
The law of conservation of energy is more widely used in engineering applications such as Hydraulics and Thermodynamics