Home Energy Auditing

Finding Out How Much Energy Appliances Are Using in the Home

Images from Wikipedia.com. Compiled by author.

Introduction

The modern home of the industrialized world has a large variety of appliances which serve many purposes. Electricity is used to cook our food and keep it fresh. We use electricity to keep our home warm or cool and our lights on. It is used to entertain us and to help us do work. But where does all of this electrical energy come from and what are the associated costs? There are financial costs which are evident when we receive a bill from the utility company. A kilowatt hour (to be discussed a little bit later) might only cost around 10 cents, but the costs can add up quick and many consumers often have trouble with their utility bills. Often overlooked however, are the environmental costs of producing the energy with technologies like coal, oil, gas, hydroelectric, and nuclear power. There are also social costs, like black outs and brown outs, associated with too much strain being placed on the electrical grid.

One way to start reducing these various costs is to start becoming more familiar with how much energy our individual appliances are using and then ask ourselves if we really need to use all of the appliances we use as often as we do.

The purpose of this website is to inform how electricity is measured as a unit of power and how to measure these units in the home. The consumer can then study their utility bill and calculate the amount of load that each appliance uses. A links page has been included to direct those who are interested to learn more about conservation and electricity.

What's a Watt?

The utility companies use a measurement known as a kilowatt hour to measure home electricity use. A kilowatt is equal to one thousand watts*. A kilowatt hour is the amount of kilowatts being consumed multiplied by one hour. For example, the amount of kilowatt hours a 1500 watt space heater running for one hour uses can be found by multiplying 1.5 kilowatts** by one hour. This space heater uses 1.5 kWh every hour it runs.    

 *  - For further information on what exactly a watt is, including a description of units of power, see this site.

** - (1500 watts) X (1 kilowatt/1000 watts)=1.5 kW.

Measuring Kilowatt Hours (kWh)

On every electricity bill consumers receive, the amount of kilowatt hours (kWh) used will be included for that bill's statement period. The resulting amount owed to the utility is mostly a computation of this number by the price per kWh. The price per kWh varies depending on region, but will usually fall between $0.06 and $0.20. Simply put, the less kWh one consumes, the lower the energy bill.

Because kWh are the standard for electric utilities in the United States, it is necessary to learn how to measure them when comparing appliances and seeing how they impact one's electric bill.

One way to measure kWh and watts is to measure with a kilowatt meter. Kilowatt meters range in price from $30 USD to $150 USD and can be found at many local hardware stores. These devices usually measure volts, amps, watts, and kWh. Figure a is a picture of one of these devices. There are a few testing techniques described below.

Constant Load Appliances For an appliance such as a space heater or a toaster, one can test the wattage and then estimate how  often the appliance is used and for how long. If a toaster is running at 300 watts for example, it is possible to come up with an estimate for how much energy it uses for a whole month. Let's assume the toaster is used about ten minutes on an average day. Since there are thirty days in a month, the toaster is operated roughly 300 minutes every month. To find kWh for the month multiply hours by watts. 300 minutes is five hours, and five hours multiplied by the 300 watts it uses equals 1500 watt * hours, or 1.5 kWh. If a kWh cost $0.12, this toaster costs $0.18/month to operate. Constant Load Appliances that Run on Cycles Other appliances may call for different measuring strategies. For example a washer and dryer use a lot of energy and run on cycles, so it may be easiest to see how many kWh they use per cycle. The dryer that was tested for research of this website for example ran at 1500 watts, or 1.5Kwh. If this dryer is used 15 times a month, for hour and a half cycles, that would account for 33.75 kWh month, which is significantly higher than what a toaster uses and may warrant a clothes line on dry days. Variable Load Appliances For an appliance like a refrigerator which doesn't run continuously, one may want to see how much it consumes in a one day cycle. To do this, plug the device into the kilowatt meter and measure the consumption for a 24 hour period. Multiply this by 30 to see the usage for a month. This should give a good idea on how much juice more variable appliances use.
		Figure a: Shows a home kWh meter (permission pending)

Other Measuring Techniques

There is another way to test the energy usage and that is by knowing the volts and the amperage. According to Ohm's law (see links page) volts multiplied by amperage will give the wattage, and methods described above can be used to figure out what this wattage means for month long usage. One benefit of this method is multi-meters that test amps and volts may be easier to find at a local hardware store than kilowatt and watt meters.

Some household energy using appliances such as light bulbs already state the amount of watts they use, and the only step needed is to figure out the amount of time that particular light is on for a one month period.

It should be noted that a lot of this type of figuring is estimation in order to get an idea of which appliances use more energy. As this data is collected, the consumer may be interested to compile this data in a spreadsheet or with paper and a pencil to add up the numbers. This way the various appliances can be compared to each other and the next step can be taken.

Conservation

Once all of the data has been compiled one can compare the numbers with a past billing statement. Questions should be asked about which appliances are using the majority of the energy? Which appliances can be done away with? For example, the dryer, which consumes much electricity, may not be necessary. Evaporation is a spontaneous process and doesn't need to be done with electrical energy. It can be done with a clothes line. Perhaps some appliances that are necessary, like a refrigerator, can be replaced with newer energy efficient designs. Make sure to recycle the old ones however at a local center because of the hazardous materials they may contain.

It may become evident that there are some holes in the energy analysis and that the numbers don't quite match the billing statement. Remember, what is being done here is rough estimation. Appliances like hot water heaters are harder to test and may be the culprit for unaccounted for kWh. These can be replaced too. Heaters and air conditioning are also difficult to test with these meters and use large amounts of electricity. One suggestion would be to  use energy saving devices like thermal curtains.

It is also possible that energy is being lost in transmission or the meter that the utility company is being read wrong. In this case, one should call their electric company.

Links

For information about home energy auditing and conservation, as well as some of the science behind these various tools and formulas, please see the links page.

References

Harmsworth, A.P. (2000). GCSE Physics: Electricity.  from http://gcse.com/electricity.htm.

Marshall, Thomas H. (1925). James Watt. Edinburg, UK: Leonard Parsons Ltd.

Author Information

This page was designed by Michael Dieter, an environmental science student at Humboldt State University.

Michael can be reached at mrd26@humboldt.edu