  We present measurements of slice energy spread at the injector section of the
European X-Ray Free Electron Laser for an electron bunch with charge of 250 pC.
Two methods considered in the paper are based on measurements at the dispersive
section after a transverse deflecting structure (TDS). The first approach uses
measurements at different beam energies. We show that with a proper scaling of
the TDS voltage with the beam energy the rms error of the measurement is less
than 0.3 keV for the energy spread of 6 keV. In the second approach we
demonstrate that keeping the beam energy constant but adjusting only the optics
we are able to simplify the measurement complexity and to reduce the rms error
below 0.1 keV. The accuracy of the measurement is confirmed by numerical
modelling including beam transport effects and collective beam dynamics of the
electron beam. The slice energy spread measured at the European XFEL for the
beam charge of 250 pC is nearly 3 times lower as the one reported recently at
SwissFEL for the same cathode material and the beam charge of 200 pC.
