How to Figure Out the Power Ratings of an Appliance

Knowing an appliance's power rating helps in determining operating costs.

Electrical current has three variables: watts, volts and amps (the total power the unit consumes). Any variable can be calculated by knowing the other two, using the formula: watts ÷ volts = amps. All new electrical devices are labeled with all three of the unit's specific power consumption figures. If the power consumption label is missing, the power ratings of an appliance can be determined with a voltmeter and a little math.

Things You'll Need

• Clamp-on voltage meter

Instructions

1. Determine the wattage of the appliance. Set the voltmeter to the correct voltage for the unit. For smaller household appliances -- such as toaster ovens, microwaves and refrigerators -- set the voltage meter to 120 volts. Larger appliances, like kitchen ranges and clothes dryers, run on 240 volts.

2. Plug the appliance into an outlet. Place the clamp of the voltage meter around the appliance's power cord according to the specifications in the meter's instruction manual. Push the wattage function on the meter and observe the reading.

3. Calculate amperage consumption using the formula: watts ÷ volts = amps. Divide the wattage reading from the voltmeter by the number of volts of the system the appliance is connected to. For example, if the wattage reading for a microwave oven is 500 watts, divide 500 by 120 volts for a resulting amperage draw of 4.2.

Tips & Warnings

• Some older clamp-on voltage meters require the meter's clamp to be placed over a voltage separator, inserted between the outlet and the appliance's plug. If the meter shows "0" when placed over the appliance cord, a voltage separator is required.

References

• "Wiring 1-2-3"; Steve Corey; 2005
• "Major Appliances"; Ron Hazelton; 1998

Resources

• Atlantech Solar: How to Easily Calculate the Conversion of Volts -- Amps & Watts