  An exact expression is derived for the time-averaged electromagnetic energy
within a magneto-dielectric coated sphere, which is irradiated by a plane and
time-harmonic electromagnetic wave. Both the spherical shell and core are
considered to be dispersive and lossy, with a realistic dispersion relation of
an isotropic split-ring resonator metamaterial. We obtain analytical
expressions for the stored electromagnetic energies inside the core and the
shell separately and calculate their contribution to the total average energy
density. The stored electromagnetic energy is calculated for two situations
involving a metamaterial coated sphere: the dielectric shell and dispersive
metamaterial core, and vice-versa. An explicit relation between the stored
energy and the optical absorption efficiency is also obtained. We show that the
stored electromagnetic energy is an observable sensitive to field interferences
responsible for the Fano effect. This result, together with the fact that the
Fano effect is more likely to occur in metamaterials with negative refraction,
suggest that our findings may be explored in applications.
