  The brightest gamma-ray burst ever observed, long-duration GRB 221009A, was
detected by GRBAlpha nano-satellite without saturation. We present light curves
of the prompt emission in 13 energy bands, from 80 keV to 950 keV, and perform
a spectral analysis to calculate the peak flux and peak isotropic-equivalent
luminosity. Since the satellite's attitude information is not available for the
time of this GRB, more than 200 incident directions were probed in order to
find the median luminosity and its systematic uncertainty. We found that the
peak flux in the $80-800$ keV range (observer frame) was
$F_{\rm{ph}}^{\rm{p}}=1300_{-200}^{+1200}$ ph cm$^{-2}$s$^{-1}$ or
$F_{\rm{erg}}^{\rm{p}}=5.7_{-0.7}^{+3.7}\times10^{-4}$ erg cm$^{-2}$s$^{-1}$
and the fluence in the same energy range of the first GRB episode lasting 300
s, which was observable by GRBAlpha, was $S=2.2_{-0.3}^{+1.4}\times10^{-2}$ erg
cm$^{-2}$ or $S^{\rm{bol}}=4.9_{-0.5}^{+0.8}\times10^{-2}$ erg cm$^{-2}$ for
the extrapolated range of $0.9-8,690$ keV. We infer the isotropic-equivalent
released energy of the first GRB episode to be
$E_{\rm{iso}}^{\rm{bol}}=2.8_{-0.5}^{+0.8}\times10^{54}$ erg in the $1-10,000$
keV band (rest frame at $z=0.15$). The peak isotropic-equivalent luminosity in
the $92-920$ keV range (rest frame) was
$L_{\rm{iso}}^{\rm{p}}=3.7_{-0.5}^{+2.5}\times10^{52}$ erg s$^{-1}$ and the
bolometric peak isotropic-equivalent luminosity was
$L_{\rm{iso}}^{\rm{p,bol}}=8.4_{-1.5}^{+2.5}\times10^{52}$ erg s$^{-1}$ (4 s
scale) in the $1-10,000$ keV range (rest frame). The peak emitted energy is
$E_p^\ast=E_p(1+z)=1120\pm470$ keV. Our measurement of
$L_{\rm{iso}}^{\rm{p,bol}}$ is consistent with the Yonetoku relation. It is
possible that, due to the spectral evolution of this GRB and orientation of
GRBAlpha at the peak time, the true values of peak flux, fluence,
$L_{\rm{iso}}$, and $E_{\rm{iso}}$ are even higher. [abridged]
