Left: effective area of H.E.S.S. II-CT5 in monoscopic mode as a function of energy for different zenith angles: the specific analysis used here has been developed to yield a large effective area near threshold, i.e. 2.6  ×  10² m² at 10 GeV and 20° zenith angle. There is a strong dependency of the effective area near the threshold on zenith angle, e.g. a drop of a factor ∼10 below 30 GeV when comparing 20° and 40° zenith angles. Right: distribution of (E_rec − E_true)/E_true as a function of E_true at 20° zenith for a power-law distribution between 5 and 120 GeV with index Γ  =  4. The parameters E_rec and E_true stand for reconstructed and simulated energies, respectively. Error bars show the spread (RMS) of events around the average value. We note that bins are correlated.