Relativity, please help

Question

Suppose that total mass of 1kg is transformed into energy, how large is this energy in kilo-watt hours.

I know the answer , 2.5*10^10 K.W.H

Answer 1

well since E = MC^2,

E = 1 x 9x10^16

   = 9x10^16J

Since watts just means Joules per second, you divide the 9x10^18 by 60^2, then you have watt hours, then divide that answer by 1000 and you have kilo-watt hours.

Answer 2

1kg  of mass corresponds to an energy of

E=mc^2 = 1*(3*10^8)^2= 9*10^16J

1 kilowatt heure correspond to an energy of 1000 j/s during 1 hour

as 1 hour =3600s  

1kWh= 3600*1000= 3.6*10^6 J

and 9*10^16/3.6*10^6 = 2.5*10^10