  Context. Arrays of radio antennas have proven to be successful in
astroparticle physics with the observation of extensive air showers initiated
by high-energy cosmic rays in the Earth's atmosphere. Accurate determination of
the energy scale of the primary particles' energies requires an absolute
calibration of the radio antennas for which, in recent years, the utilization
of the Galactic emission as a reference source has emerged as a potential
standard. Aims. To apply the "Galactic Calibration", a proper estimation of the
systematic uncertainties on the prediction of the Galactic emission from sky
models is necessary, which we aim to determine on a global level as well as for
the specific cases of selected radio arrays. We further aim to quantify the
influence of the quiet Sun on the Galactic Calibration. Methods. We look at
four different sky models that predict the full-sky Galactic emission in the
frequency range from 30 to 408 MHz and compare them. We make an inventory of
the reference maps on which they rely and use the output of the models to
determine their global level of agreement. Next, we take the sky exposures and
frequency bands of selected radio arrays into account and repeat the comparison
for each of them. Finally, we study the relative influence of the Sun in its
quiet state by projecting it onto the sky with brightness data from recent
measurements. Results. We find systematic uncertainty of 12% on the predicted
power from the Galactic emission, which scales to approximately half of that
value as the uncertainty on the determination of the energy of cosmic
particles. When looking at the selected radio arrays, the uncertainty on the
predicted power varies between 10% and 19%. The influence of the quiet Sun
turns out to be insignificant at the lowest frequencies but increases to a
relative contribution of ~ 20% around 400 MHz.
