Question

Electricity from the local power company costs 5.28 cents/kWh. A homeowner wishes to pay a maximum of 30 cents per month for the electricity to power a 120-W stereo system. For how many hours per month could the stereo system be used?

Answer 1

The maximum cost allowed in one month is 30 cents. Since electricity costs 5.28 cents/kWh, the maximum energy that can be used is

`E_(max) = (30 cents)/(5.28 cents/kWh)= 5.68 kWh=5680 Wh`

The stereo system uses a power of P=120 W. Since the power is the energy per unit of time:

`P= (E)/(t)`
the stereo system can be used for a number of hours equal to:

`t= (E)/(P)= (5680 Wh)/(120 W)=47.3 h`