Assume that a certain city consumes electrical energy at an average rate of 2.0× 10^9 W. What would be the mass change

Question

in producing enough energy to keep this city running for 21 weeks?

Answer 1

E = m c^2
so
m = E/c^2

E = 2*10^9 *21 * 7 *24 *3600 = 2.54*10^16
c = 3*10^8 m/s
c^2 = 9 * 10^16

so
m = 2.54 / 9 = .282 Kg = 282 grams