  A simulation study of the energy released by extensive air showers in the
form of MHz radiation is performed using the CoREAS simulation code. We develop
an efficient method to extract this radiation energy from air-shower
simulations. We determine the longitudinal profile of the radiation energy
release and compare it to the longitudinal profile of the energy deposit by the
electromagnetic component of the air shower. We find that the radiation energy
corrected for the geometric dependence of the geomagnetic emission scales
quadratically with the energy in the electromagnetic component of the air
shower with a second-order dependence on the atmospheric density at the
position of the maximum shower development $X_\mathrm{max}$. In a measurement
where $X_\mathrm{max}$ is not accessible, this second order dependence can be
approximated using the zenith angle of the incoming direction of the air shower
with only a minor loss in accuracy. Our method results in an intrinsic
uncertainty of 4% in the determination of the energy in the electromagnetic
air-shower component, which is well below current experimental uncertainties.
