  We consider a general two-component plasma of classical pointlike charges
$+e$ ($e$ is say the elementary charge) and $-Z e$ (valency $Z=1,2,\ldots$),
living on the surface of a sphere of radius $R$. The system is in thermal
equilibrium at the inverse temperature $\beta$, in the stability region against
collapse of oppositely charged particle pairs $\beta e^2 < 2/Z$. We study the
effect of the system excess charge $Q e$ on the finite-size expansion of the
(dimensionless) grand potential $\beta\Omega$. By combining the stereographic
projection of the sphere onto an infinite plane, the linear response theory and
the planar results for the second moments of the species density correlation
functions we show that for any $\beta e^2 < 2/Z$ the large-$R$ expansion of the
grand potential is of the form $\beta\Omega \sim A_V R^2 + [\chi/6 - \beta
(Qe)^2/2] \ln R$, where $A_V$ is the non-universal coefficient of the volume
(bulk) part and the Euler number of the sphere $\chi=2$. The same formula,
containing also a non-universal surface term proportional to $R$, was obtained
previously for the disc domain ($\chi=1$), in the case of the symmetric $(Z=1)$
two-component plasma at the collapse point $\beta e^2=2$ and the jellium model
$(Z\to 0)$ of identical $e$-charges in a fixed neutralizing background charge
density at any coupling $\beta e^2$ being an even integer. Our result thus
indicates that the prefactor to the logarithmic finite-size expansion does not
depend on the composition of the Coulomb fluid and its non-universal part
$-\beta (Qe)^2/2$ is independent of the geometry of the confining domain.
