The solution of Lyapunov equations can be represented as a sum of Hermitian matrices corresponding either to particular eigenvalues of the system matrix, or to pairwise combinations of these eigenvalues. These eigen-parts or sub-Gramians proved to be useful for the stability analysis of electric power systems. In this paper we compare and contrast the sub-Gramians and participation factors as applied to the power system state estimation. Using the sub-Gramian approach we introduce the energy participation factor as a new indicator for selective modal analysis. For a stable system it characterizes the participation of i-th mode and initial k-th state in the integrated energy produced in k-th state. We explain the conceptual meaning and practical usefulness of energy participation factors and contrast them with the conventional participation factors in a selective modal analysis of the IEEE 57-bus test model.