How to pick appliances by calculating how much energy they will use
Are you considering buying a new electrical appliance? All appliances, including refrigerators, air conditioners, electric motors, and portable electric heaters, have one commonality — they have a nameplate. Many of you probably don’t pay much attention to the plate, but you can actually learn a lot from it. The information on the plate includes the model and serial number, the manufacturing date, and the location of assembly, as well as other useful information.
For someone like me, who deals with energy and energy use questions on a regular basis, the most important information found on this plate is the watts, volts, or amps. If one or more pieces of this information are on the plate, you can get a pretty good idea of how much energy the device or appliance will use over a period of time.
Let’s take a look at a couple of examples and do the math together, starting with a portable infrared heater, which is a device that I often find in many of our members’ homes.
We see the model and serial number and a power supply of 120 volts. It can be plugged into a regular household outlet, and it uses 1,500 watts. Here’s how the 1,500 watts can be converted into kilowatt hours (kWh):
- 1,500 watts/1,000 = 1.5 kW
- 1.5 kW x 10 (hours run per day) = 15 kWh/day
- 15 x 30 days in a month = 450 kWh/month
- 450 kWh x $.10 per kWh = $45 per month if the heater runs 10 hours a day, every day
The number of kWh, and therefore the cost, can add up quickly when a homeowner lets this type of heater run all day long — or if you have more than one heater.
Now, how about an appliance in almost every home: a refrigerator? This Frigidaire unit has a rated current of 3.3 amps.
Let’s start with a simple equation to convert amps to watts: amps x volts = watts.
3.3 amps x 120 volts = 396 watts
If the refrigerator runs a total of five hours a day, every day for a month, how much does it end up costing you?
- 396 watts/1000 = .396 kW
- .396 kW x 5 hours = 1.98 kWh/day
- 1.98 kWh/day x 30 days = 59.4 kWh/month
- 59.4 kWh x $.10 per kWh = $5.94/month
Where does extra cost from a refrigerator come from?
Older model units, which run at a higher amp draw, are occasionally moved into the garage or basement instead of being thrown out or recycled. Garages or basements are often unconditioned spaces affected by the outdoor temperatures and elements.
Central AC or heat pump
One final example, which can be found in a majority of homes today, is a central air conditioner or heat pump. The nameplate pictured is from a 3-ton Maytag unit. In this example, we will focus on the run loading amps (RLA). On this unit, it is rated at 15.2 amps. Now that we have our amps, we can run the numbers — let’s determine the cost of a system running eight hours per day.
- 15.2 amps x 220 volts = 3,344 watts
- 3,344 watts/1000 = 3.344 kW
- 3.344 kW x 8 hours = 26.75 kWh/day
- 26.75 kWh/day x 30 days = 802.6 kWh/month
- 802.6 kWh x $.10 per kWh = $80.26/month
Weather, condition of the house, thermostat setting, number of people in a home, and proper insulation and air-sealing are a few variables that could cause cost and the operation of an air conditioner or heat pump to vary.
Due to variable conditions in each home, these calculations are estimates that can help you determine an approximate cost of various appliances and devices in your home. Nameplates on appliances and devices are a great place to start when trying to determine energy costs, so don’t overlook them as useless information. They could help you find your home’s hidden high energy user.
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