From Einstein's mass-energy equivalence relation, the amount of matter converted into energy is given by
$\Delta E=\Delta m c^{2}$
Given, $\Delta m=1\, mg =10^{-6} kg$,
$c=3 \times 10^{8} m / s$
$\therefore \Delta E=\left(10^{-6}\right)\left(3 \times 10^{8}\right)^{2}$
$\Rightarrow \Delta E=9 \times 10^{10} J$