Your electricity bill shows that you use 600 kWh of electricity each month. Your refrigerator, which is 15 years old, could be responsible for up to 25 percent of this electricity consumption. Newer refrigerators are more efficient, meaning that they use less energy to do the same amount of work.

If you wish to conserve electrical energy, should you replace your refrigerator? How can you compare the efficiency of your old refrigerator with that of more efficient newer models?

Your refrigerator uses 500 watts when the motor is running. The motor runs for about 30 minutes per hour (or a total of 12 hours per day).

1. Start by calculating the amount of energy your current refrigerator uses.
\[
0.5 \text{ kW} \times 12 \text{ hours/day} = 6 \text{ kWh/day}
\]
\[
6 \text{ kWh/day} \times 365 \text{ days/year} = 2,190 \text{ kWh/year}
\]

2. How much more efficient is the best new refrigerator compared with your older model?
The best new model uses 400 kWh per year.
\[
\text{Your refrigerator uses 2,190 kWh per year.}
\]
\[
2,190 \text{ kWh/year} - 400 \text{ kWh/year} = 1,790 \text{ kWh/year}
\]

3. Assume that you are paying, on average, $0.10 per kilowatt-hour for electricity. A new refrigerator would cost you $550. You will receive a rebate of $50 from your electric company for purchasing an energy-efficient refrigerator. If you replace your refrigerator, how long will it be before your energy savings compensate you for the cost of the new appliance?

\[
\text{You will save } 1,790 \text{ kWh/year} \times \$0.10/\text{kWh} = \$179/\text{year}
\]
\[
\text{Dividing \$500 by \$179 indicates that in less than 3 years, you will recover the cost of the new appliance.}
\]

**Your Turn**

Environmental scientists must often convert energy units in order to compare various types of energy. For instance, you might want to compare the energy you would save by purchasing an energy-efficient refrigerator with the energy you would save by driving a more fuel-efficient car.

Assume that for the amount you would spend on the new refrigerator ($500), you can make repairs to your car engine that would save you 20 gallons (76 liters) of gasoline per month. (Note that 1 L of gasoline contains the energy equivalent of about 10 kWh.)

Using this information and Table 5.1 on page 44, convert the quantities of both gasoline and electricity into joules and compare the energy savings. Which decision would save the most energy?