During 3 hours one winter afternoon, when the outside temperature was 11°C, a house heated by electricity was kept at 20°C with the expenditure of 54 kwh (kilowatt·hours) of electric energy.

What was the average energy leakage in joules per second (watts) through the walls of the house to the environment (the outside air and ground)?

Explanation:

Since the temperature difference between inner and outside temperature remains constant , condition of steady state has been achieved . So heat produced is equal to heat leaked

heat leaked = heat produced

heat produced  = 54 kwh = 54 x 1000 x  60 x 60 J  

time taken  = 3 x 60 x 60 s

heat produced per second

= 54 x 1000 x  60 x 60 / 3 x 60 x 60

= 18000 J /s

heat leaked = 18000 J /s

change tension