  Our objective is to critically assess the X-ray flux variability as a tool
for measuring the black hole (BH) mass in active galactic nuclei (AGN). We aim
to establish a prescription for estimating BH masses based on measurements of
the normalised excess variance from X-ray data. We discuss the minimum
requirements in terms of the light-curve duration and X-ray signal-to-noise
ratio (S/N) to enable a reliable determination that is comparable to what can
be derived from the continuum and emission line reverberation studies. We used
the light curves of local Seyfert from the Nuclear Spectroscopic Telescope
Array hard X-ray mission (NuSTAR), to compute the normalised excess variance
(NXV) in the 3-10 and 10-20 keV bands, thus extending the analysis to an energy
band higher than 10 keV. The excess variance measurements were then combined
with independent BH mass estimates from the literature to establish the MBH
versus NXV relation for different samples and weigh its accuracy in terms of
the light-curve duration and X-ray S/N. We find that it is possible to
accurately measure the BH mass in AGN using excess variance measurements in the
3-10 and the 10-20 keV bands, however, strong quality requirements should be
applied. The minimum necessary S/N and duration of the light curves used to
compute the excess variance ought to be 3 and approximately 100 ks,
respectively. We provide a linear relationship between the normalised excess
variance and the black hole mass that can be used to estimate the latter, with
an average uncertainty of the order of 0.4 to 0.25 dex (depending on the
adopted light-curve segment duration).
