Reviewed October 1993

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Contents

• How to compute operation costs
• Conserving energy

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Computing the Cost of Using Home Appliances

Marilyn W. Caselman
Department of Environmental Design

Utility bills are rising across most parts of Missouri. Living costs have followed suit. Families look for ways to save on expenses, and utility bills come under scrutiny. This guide suggests possible ways to increase energy efficiency in home appliances.

How to compute operation costs

Computing the cost of operation of appliances may help the consumer:

• Become more aware of their use of utilities;
• Do their part to conserve energy; and
• Consider ways to cut this use if their costs are beyond their budget.

Costs can be figured with the following information:

• Number of kilowatts used.
• Length of time the appliance is used.
• Cost of electricity per kilowatt hour.
• Percentage of time used, if unit cycles on and off (such as a thermostatically controlled appliance).

Some of this information can be found on the nameplate of the appliance and/or in the use and care instructions. Check with a local supplier for current energy costs.

The following information may help interpret nameplate information and utility bills into a usable form:

• W (watts) = V (volts) x A (amperes)
• 1,000 W = 1 KW (kilowatt)
• 1 hp (horsepower) = 746 W
• 1 KW x 1 hr (hour) = 1 KWH (kilowatt hour)

The following example shows how to compute the cost of using a given appliance.

A blender rated at 500 watts is used four hours a month at a cost of $0.07 per kilowatt hour.

• 500 W = 0.500 KW
• 0.500 KW x 4 hours = 2 KWH
• 2 KWH x 0.07/KWH = $0.14 cost of operation for the month

A one-fourth horsepower washing machine is used two hours a week. What would the monthly cost at a $0.07 per kilowatt hour rate be?

A typical efficiency for a fractional horsepower, single-phase electric motor is 60 percent. Since electric motors are rated according to their output, the input to a 1/4 horsepower electric motor would be

• 0.4167 hp = 311 Watts
• 311 Watts = 0.311 KW
• 2 hours per week x four weeks = 8 hours
• 0.311 KW x 8 hours = 2.488 KWH
• 2.488 KWH x $0.07 = $0.1742 cost of monthly operation

The on-off cycling in thermostatically controlled appliances ranges from 50 to 80 percent actual "on" time. Since this figure is not always available, 66.6 percent (two-thirds) can be used as an estimate.

For example, a fry pan rated at 1,000 watts is used three hours at $0.07 per kilowatt hour.

• 1,000 W = 1.0 KW
• 1.0 KW x 3 hours = 3.0 KWH
• 2/3 x 3.0 KWH = 2 KWH (allowing for on-off cycling)
• 2 KWH x 0.07 = $0.14 cost of operation

An air conditioner is rated 4,000 watts. Consider August in Missouri with 20 days of operation. Use the two-thirds off and on time or 16 hours a day, and the seven cents per KWH rate.

• 4,000 watts = 4 KW
• 4 KW x 16 x 20 days = 1280 KWH
• 1280 KWH x 0.07 = $89.60

For more accuracy, the horsepower of the fan motor would need to be added to this. Assuming it is a one-half horsepower and that it has an efficiency of 60 percent, the input horsepower to the motor would be

• 0.8333 hp = 0.622 KW
• 0.622 x 16 x 20 = 199 KWH
• 199 x 0.07 = $13.93

The total monthly cost would be $103.53. Thus, an appliance that heats or cools and also has a motor is more expensive than one that only heats, cools or is motor driven.

Conserving energy

Although appliances are said to use less than 5 percent of the nation's total energy, this percentage can decrease by using appliances wisely. Below are a few suggestions.

• Don't overload an electric circuit with high-wattage appliances. The normal limit for an electric circuit of 15 amps is 1600 watts. An overloaded circuit may blow the fuse or trip the circuit breaker as well as make appliances on the circuit operate inefficiently.
• Use appliances only for the purpose intended.
• Appliances should be cared for as the directions in the operations manuals indicate. This adds to safety, efficiency and life of the appliance.
• Avoid using extension cords with appliances. This cuts the fuel efficiency of the equipment.
• Turn an appliance off when not in use.
• Unplug cords after use. This can prevent accidents if the temperature control is bumped to the "on" position.

GH4878, reviewed October 1993