Explanation:
According to Einstein's mass-energy equivalence relation
\(E = m{c^2} = \left( {0.5 \times {{10}^{ - 3}}kg} \right){\left( {3 \times {{10}^8}m{s^{ - 1}}} \right)^2}\)
\(\,\,\,\, = 4.5 \times {10^{13}}J\)
\(\,\,\,\, = \frac{{4.5 \times {{10}^{13}}}}{{3.6 \times {{10}^6}}}kWh\,\,\,\left( {\because 1\,kWh = 3.6 \times {{10}^6}J} \right)\)
\(\,\,\,\, = 1.25 \times {10^7}kWh\)