  A novel interferometric method is presented for the measurement of the
absolute energy of electron beams. In the year 2016, a pioneering experiment
was performed using a 195 MeV beam of the Mainz Microtron (MAMI). The
experimental setup consisted of two collinear magnetic undulators as sources of
coherent optical synchrotron light and a high-resolving grating monochromator.
Beam energy measurements required the variation of the relative undulator
distance in the decimeter range and the analysis of the intensity oscillation
length in the interference spectrum. A statistical precision of 1 keV was
achieved in 1 hour of data taking, while systematic uncertainties of 700 keV
were present in the experiment. These developments aim for a relative precision
of $10^{-5}$ in the absolute momentum calibrations of spectrometers and
high-precision hypernuclear experiments. Other electron accelerators with beam
energies in this regime such as the Mainz Energy Recovering Superconducting
Accelerator (MESA) might benefit from this new method.
