  Microwave Kinetic Inductance detectors (MKIDs) have been recognized as a
powerful new tool for single photon detection. These highly multiplexed
superconducting devices give timing and energy measurement for every detected
photon. However, the full potential of MKID single photon spectroscopy has not
been reached , the achieved energy resolution is lower than expected from first
principles. Here, we study the efficiency in the phonon downconversion process
following the absorption of energetic photons in MKIDs. Assuming previously
published material properties, we measure an average downconversion efficiency
for three TiN resonators is $\eta$=0.14. We discuss how this efficiency can
impact the intrinsic energy resolution of MKID, and how any uncertainty in the
unknown density of electron states at the Fermi energy directly affects the
efficiency estimations.
