  A stationary physical system satisfies peculiar balance conditions involving
mean values of appropriate observables. In this paper we show how to deduce
such quantitative links, named balance equations, demonstrating as well their
usefulness in bringing to light physical properties of the system without
solving the Schrodinger equation. The knowledge of such properties in the case
of Rabi Hamiltonian is exploit to provide arguments to make easier the
variational engineering of the ground state of this model.
