Energy Use Investigation 6.2:

What Are We Paying For?

Determine the costs of various home uses of electricity, learn what your biggest electricity eaters are, and find out where the money goes.

The energy E (in watt-hours) an appliance uses is its wattage P (in watts) times the amount of time T (in hours) it is on.
E = P x T

The power P (in watts) of any electrical device is the product of the voltage V (in Volts) and the current I (in amperes).
P = V x I


• A household electric bill for the month

Strategies for Investigation

  1. Prepare a table with five columns like the sample below. In column 1 list all the electrical appliances in your home. Group the appliances in categories such as heating, lighting, food preparation, washing clothes, power tools, entertainment. Make the list as complete as you can.

  2. In column 2 list the power rating, in watts, of each device. Most of them have a power rating label on the back or bottom. If the rating is given in amperes for a wall-socket appliance, you must multiply the ampere rating by 120 volts to figure out the power rating in watts.

  3. In column 3 estimate the average number of hours per day the device is on. A clock would be on all the time. Some devices like a heater or refrigerator are plugged in all the time but switch on and off automatically. Three hours a day is about right for most household refrigerators. For items that are not used every day, estimate the use for a week and divide by 7.
Sample chart of typical electrical costs in one month
SAMPLE CHART: If your home’s water heater is gas-fueled, its cost does not appear on the electric bill. If your home is rented, you may not pay the direct cost of heating water. How is that cost passed to the people in your home?
  1. Column 4 is for the energy used by each device each day. Find the energy by multiplying the power (P) and the time (T) for each appliance. That gives you energy in watt-hours. Divide by 1,000 to get kilowatt-hours.

  2. Column 5 is for the monthly energy cost. Find out the cost or rate (R) that you pay per kilowatt-hour by looking at an electricity bill. For each appliance, multiply the energy usage by the cost per kilowatt-hour to obtain the actual cost of each item per day. Multiply by 30 to get the monthly cost.


Analysis of Results

  1. Add up the costs for the appliances you listed to get an idea of your monthly electrical costs. Does it nearly match your electric bill? If not, modify your estimates until you have a reasonable idea of costs for each category of energy use.

  2. Which items and categories of appliances were the biggest electricity eaters? Display the results as a bar graph or a pie chart. You need not include each small item separately, for instance all light bulbs and lamps can be lumped together.

  3. Based on your analysis, can you make any recommendations about how to save money on energy costs?