Redshift is one of the most fundamental observational phenomena in physics. In Energy-Efficiency Theory (EET), we interpret redshift as a projection of constrained-state energy distribution onto observed space. Starting from the three axioms, we define the constrained potential \(\mathcal{U}(\mathbf{r})\) as the spatial density of constrained-state energy. In the weak-field, static limit, the gravitational potential is given by \(\Phi(\mathbf{r}) = -G \int \mathcal{U}(\mathbf{r}')/|\mathbf{r}-\mathbf{r}'|\,d^3r'\). The standard redshift formula \(1+z = \sqrt{g_{00}(R)/g_{00}(E)}\) with \(g_{00}=1+2\Phi/c^2\) follows from the concept of observed space. We show that for spherically symmetric objects, the exterior gravitational field is uniquely determined by the total mass, in agreement with Birkhoff's theorem; therefore EET and general relativity give identical predictions in this case. For non-spherical systems (e.g., rotating neutron stars, binary mergers), small corrections may arise from higher multipole moments. These possibilities are discussed qualitatively as directions for future quantitative study. This paper provides an energy-ontological foundation for redshift without claiming new empirical predictions; it serves as a conceptual anchor within the EET framework.