Variability of Galactic cosmic ray (GCR) is often expressed in terms of the modulation potential, which is typically assessed using energy-integrating ground-based detectors, such as neutron monitors (NMs) for the last decades or cosmogenic isotopes on the time scales of centuries and millennia.
In order to estimate the energy dependence of the GCR variability we re-assess here the effective energy $E_{\rm eff}$ of each type of detector, which is defined so that the variability of the GCR particles at this energy is equal to that of the detector's count rate.
We found that $E_{\rm eff}$ is 11--12 GeV/nuc for the standard polar sea-level neutron monitor, but it is essentially smaller for cosmogenic isotopes, being 6--7 GeV/nuc for $^{14}$C and 5.5--6 GeV/nuc for $^{10}$Be, respectively.
It is also discussed that this effective energy is robustly defined and is hardly dependent on the primary assumptions on the local interstellar spectrum of GCR.