Internal photoemission of electrons from uncapped monolayer graphene to insulating SiO2 has been observed in samples prepared by water-intercalation based graphene transfer. The barrier height between the graphene Fermi level and the oxide conduction band bottom was reproducibly found to be 4.1-4.2 eV. Moreover, this value was weakly sensitive to the contacting metal work function (Al, Cu, Au). This barrier height corresponds to an effective work function of graphene close to 5.0 eV, which is nearly 0.5 eV higher than the usually reported vacuum value.