Question

How many kw-hr are consumed by twenty-five 60 watts light bulbs burning 5 hrs. in a 12v 

circuit?

Answer 1

The total power dissipated on the bulbs

$$P=NP_0=25*60\:\rm W=1500\: W$$

The amount of electricity

$$W=Pt\\ =1500\:\rm W*5\: hr=7500\: W\cdot hr=7.5\: kW\cdot hr$$