  Element- and site-specific resonant inelastic x-ray scattering spectroscopy
(RIXS) is employed to investigate electron correlation effects in {$\rm
NaV_2O_5$}. In contrast to single photon techniques, RIXS at the vanadium $L_3$
edge is able to probe $d-d^*$ transitions between V d-bands. A sharp energy
loss feature is observed at -1.56 eV, which is well reproduced by a model
calculation including correlation effects. The calculation identifies the loss
feature as excitation between the lower and upper Hubbard bands and permits an
accurate determination of the Hubbard interaction term $U= 3.0 \pm 0.2$ eV.
