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
$E_{max} = \frac{30 cents}{5.28 cents/kWh}= 5.68 kWh=5680 Wh$

The stereo system uses a power of P=120 W. Since the power is the energy per unit of time:
$P= \frac{E}{t}$
the stereo system can be used for a number of hours equal to:
$t= \frac{E}{P}= \frac{5680 Wh}{120 W}=47.3 h$