$E_o = 18000$ kWh

$textbf{Find:}$ mass

Using the mass-energy equivalence formula, we can find the mass, $m$ as follows:

$$
begin{align*}
E_o &= mc^2 \
m &= dfrac{E_o}{c^2} \
&= dfrac{(18000 text{ J/s})(1000)(3600 text{ s})}{(3.0 times 10^8 text{m/s})^2} \
&= 7.2 times 10^{-7} text{ kg}
end{align*}
$$

Hence, the mass required to have a rest energy of 18000 kWh is $boxed{7.2 times 10^{-7} text{ kg}}$