  By combining the results of SNO and Super-Kamiokande one can derive - in the
absence of sterile neutrinos - the total neutrino flux produced from $^8$B
decay in the Sun. We use this information to check the accuracy of several
input parameters of solar model calculations. Opacity and p-p fusion cross
section are constrained by the $^8$B flux measurement to the level of few per
cent. The central solar temperature is determined to the one-percent level. We
also find an upper limit for the flux on Earth of sterile neutrinos. We discuss
the role of nuclear physics uncertainties on these determinations.
