Estimating Appliance Energy Use and Cost of Operation

 If you’re trying to determine how much energy an appliance consumes, one of the easiest ways is to locate its wattage rating which you can usually find printed on its nameplate. The wattage listed is the maximum power drawn by the appliance. Since many appliances have a range of settings (for example, fans have slow and fast speeds), the actual amount of power consumed depends on the setting used.

If the wattage is not listed on the appliance, but the current draw (amps) is, you can calculate the wattage by multiplying the amps by the voltage used by the appliance. Most appliances in the United States use 120 volts. Larger appliances, such as clothes dryers and electric ranges, use 240 volts.

Many appliances continue to draw a small amount of power when they are switched “off.” These “phantom loads” occur in most appliances that use electricity, such as VCRs, televisions, computers, and kitchen appliances. Most phantom loads will increase the appliance’s energy consumption a few watt-hours. These loads can be avoided by unplugging the appliance or using a power strip and using the switch on the power strip to cut all power to the appliance.

If you have the wattage you can calculate the annual cost of using the appliance as follows:

Annual Kilowatt Hours (kWh) Used = ((Watts) x (Hours Used per Day) x (Days used per year))/1000

Annual Operating Cost = (Annual kWh Used) x (Cost/kWh)

The Cost/kWh can be found on your electric bill.

Television example:

A 32” flat screen TV consumes 180 Watts (from the back of my TV – see photo below) and according to Nielson Research the average family watches 8 hours and 15 minutes of television per day, and the average electric rate in the US is 12 cents/kWh (source: US Energy Information Administration report July 2010 – look up your states average rate here).

appliance energy use

Annual kWh Used = ((180 watts) x (8.25 hours /day) x (365 days / year)) / 1000 = 542 kWh/year

Annual Operating Cost = (542 kWh/year) x (0.12 dollars / kWh) = $65 / year

Dishwasher Example:

A dishwasher can consume a lot of electricity when using the heated drying option. Think about just how much heat comes out of your dishwasher if you open it just after it finishes. That heat is generated by a large electric element that you can see at the bottom of the dishwasher. The label inside the door or my dishwasher, which is only 2 years old, reads 9.6 amps at 120 volts (see photo below). It’s a fair assumption that we run it once per day.

Dishwasher Power Comsumption Plate

Watts = (Amps) x (Volts) = 9.6 amps x 120 volts = 1152 watts

Annual kWh Used = ((1152 watts) x (365 days/year))/1000 = 420 kWn/year

Annual Operating Cost = (420 kWh/year) x (0.12 dollars/kWh) = $50.40 / year

 

OK, those calculations were pretty easy but there is a faster and easier way to determine an appliance’s power consumption. You can purchase a relatively inexpensive electrical meter.

For between $20 and $30 you can buy a Kill A Watt electricity monitor or similar product from another manufacturer. Read the Kill A Watt user’s manual by clicking here. You simply plug it into an electrical outlet and then plug the appliance into it and it provides you with your energy consumption. The following is an excerpt from the user manual for the P4460 Kill A Watt EZ:

“The P4460 Kill A Watt™ EZ is an easy to use consumer power meter allowing the user to accurately measure power consumption of household appliances and to determine the actual cost of power consumed. The unit will also project, in real time, the cost of continued use of the appliance in time periods of Hour, Day, Week, Month, and Year. The P4460 features a precise circuit which provides very accurate results. Voltage and current are measured using true RMS methods. Total consumed power is displayed in Kilowatt-hours (KWH). It is easy to use with a large display and simple 5-button interface. The unit is safe as it features ETL listing and over-current protection.”

 

 kill a watt EZ meter

 

 

With an electric monitor you can also measure an appliance’s phantom electrical load which is electricity that is consumed by the appliance when it is plugged in but not in use or turned on.  The following video demonstrates just how simple it is to use one of these meters: 

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Some versions of this meter allow you to enter the electrical rate from your power company and it will calculate the cost of operating the appliance. With the power consumption numbers in hand for your existing appliances you can compare the numbers to newer Energy Star rated products or just decide that the old TV in the guest bedroom should be unplugged when it’s not in use or maybe that old refrigerator in the garage just isn’t worth the cost of operating it.